Allow restricting completion of flags

This PR allows a command to specify that flags should only be completed
when a prefix is present. The motivation behind this is to have the
initial complation to prefer displaying argument completions and only
display flag completions when the user enters "hyphen <tab>".

command.go

@@ -1,6 +1,10 @@
 package complete
 
-import "github.com/posener/complete/match"
+import (
+	"strings"
+
+	"github.com/posener/complete/match"
+)
 
 // Command represents a command line
 // It holds the data that enables auto completion of command line
@@ -19,6 +23,11 @@ type Command struct {
 	// Global flags that can appear also after a sub command.
 	GlobalFlags Flags
 
+	// FlagsRequirePrefix requires that the prefix is provided before flags are
+	// autocompleted. This allows completion to only display completions for
+	// arguments if for example no hypen is provided.
+	FlagsRequirePrefix string
+
 	// Args are extra arguments that the command accepts, those who are
 	// given without any flag before.
 	Args Predictor
@@ -86,7 +95,9 @@ func (c *Command) predict(a Args) (options []string, only bool) {
 		return predictor.Predict(a), true
 	}
 
-	options = append(options, c.GlobalFlags.Predict(a)...)
+	if c.FlagsRequirePrefix == "" || strings.HasPrefix(a.Last, c.FlagsRequirePrefix) {
+		options = append(options, c.GlobalFlags.Predict(a)...)
+	}
 
 	// if a sub command was entered, we won't add the parent command
 	// completions and we return here.
@@ -101,7 +112,9 @@ func (c *Command) predict(a Args) (options []string, only bool) {
 	}
 
 	options = append(options, c.Sub.Predict(a)...)
-	options = append(options, c.Flags.Predict(a)...)
+	if c.FlagsRequirePrefix == "" || strings.HasPrefix(a.Last, c.FlagsRequirePrefix) {
+		options = append(options, c.Flags.Predict(a)...)
+	}
 	if c.Args != nil {
 		options = append(options, c.Args.Predict(a)...)
 	}

complete_test.go

@@ -25,6 +25,13 @@ func TestCompleter_Complete(t *testing.T) {
 				},
 				Args: PredictFiles("*.md"),
 			},
+			"sub3": {
+				Flags: Flags{
+					"-flag4": PredictAnything,
+					"-flag5": PredictNothing,
+				},
+				FlagsRequirePrefix: "-",
+			},
 		},
 		Flags: Flags{
 			"-o": PredictFiles("*.txt"),
@@ -41,7 +48,7 @@ func TestCompleter_Complete(t *testing.T) {
 	}{
 		{
 			args: "",
-			want: []string{"sub1", "sub2", "-h", "-global1", "-o"},
+			want: []string{"sub1", "sub2", "sub3", "-h", "-global1", "-o"},
 		},
 		{
 			args: "-",
@@ -49,7 +56,7 @@ func TestCompleter_Complete(t *testing.T) {
 		},
 		{
 			args: "-h ",
-			want: []string{"sub1", "sub2", "-h", "-global1", "-o"},
+			want: []string{"sub1", "sub2", "sub3", "-h", "-global1", "-o"},
 		},
 		{
 			args: "-global1 ", // global1 is known follow flag
@@ -57,7 +64,7 @@ func TestCompleter_Complete(t *testing.T) {
 		},
 		{
 			args: "sub",
-			want: []string{"sub1", "sub2"},
+			want: []string{"sub1", "sub2", "sub3"},
 		},
 		{
 			args: "sub1",
@@ -67,6 +74,10 @@ func TestCompleter_Complete(t *testing.T) {
 			args: "sub2",
 			want: []string{"sub2"},
 		},
+		{
+			args: "sub3",
+			want: []string{"sub3"},
+		},
 		{
 			args: "sub1 ",
 			want: []string{"-flag1", "-flag2", "-h", "-global1"},
@@ -107,13 +118,21 @@ func TestCompleter_Complete(t *testing.T) {
 			args: "sub1 -flag2 ",
 			want: []string{"-flag1", "-flag2", "-h", "-global1"},
 		},
+		{
+			args: "sub3 ",
+			want: []string{"-h", "-global1"},
+		},
+		{
+			args: "sub3 -",
+			want: []string{"-flag4", "-flag5", "-h", "-global1"},
+		},
 		{
 			args: "-no-such-flag",
 			want: []string{},
 		},
 		{
 			args: "-no-such-flag ",
-			want: []string{"sub1", "sub2", "-h", "-global1", "-o"},
+			want: []string{"sub1", "sub2", "sub3", "-h", "-global1", "-o"},
 		},
 		{
 			args: "no-such-command",
@@ -121,7 +140,7 @@ func TestCompleter_Complete(t *testing.T) {
 		},
 		{
 			args: "no-such-command ",
-			want: []string{"sub1", "sub2", "-h", "-global1", "-o"},
+			want: []string{"sub1", "sub2", "sub3", "-h", "-global1", "-o"},
 		},
 		{
 			args: "-o ",
@@ -149,7 +168,7 @@ func TestCompleter_Complete(t *testing.T) {
 		},
 		{
 			args: "-o ./readme.md ",
-			want: []string{"sub1", "sub2", "-h", "-global1", "-o"},
+			want: []string{"sub1", "sub2", "sub3", "-h", "-global1", "-o"},
 		},
 		{
 			args: "-o sub2 -flag3 ",