Adding separate tag option

As outlined in #49, there is a need to mimic the behavior of other
applications by interweaving positional and non-positional parameters.

This change adds the 'separate' option that will force a arg of type
[]string to only read the next supplied value.

For example, when dealing with the following arg type:

var MyArgs struct {
    Pos []string `arg:"positional"`
    Separate []string `arg:"-s,separate"`
}

This commit will parse the following command line:

./app pos1 pos2 -s=separate1 -s=separate2 pos3 -s=separate3 pos4

Such that MyArgs.Pos will be [pos1 pos2 pos3 pos4] and MyArgs.Separate
will be [separate1 separate2 separate3].

Unit tests for the above have also been written and are included in this
commit, as well as the addition of a section to README.md and an example
func in example_test.go.

Fixes #49

1 files changed, 15 insertions, 0 deletions

diff --git a/example_test.go b/example_test.go
index 6fb5197..c34effa 100644
--- a/example_test.go
+++ b/example_test.go
@@ -71,6 +71,21 @@ func Example_multipleValues() {
 	fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 }
 
+// This eample demonstrates multiple value arguments that can be mixed with
+// other arguments.
+func Example_multipleMixed() {
+	os.Args = []string{"./example", "-c", "cmd1", "db1", "-f", "file1", "db2", "-c", "cmd2", "-f", "file2", "-f", "file3", "db3", "-c", "cmd3"}
+	var args struct {
+		Commands  []string `arg:"-c,separate"`
+		Files     []string `arg:"-f,separate"`
+		Databases []string `arg:"positional"`
+	}
+	MustParse(&args)
+	fmt.Println("Commands:", args.Commands)
+	fmt.Println("Files", args.Files)
+	fmt.Println("Databases", args.Databases)
+}
+
 // This example shows the usage string generated by go-arg
 func Example_usageString() {
 	// These are the args you would pass in on the command line