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, 17 insertions, 1 deletions

diff --git a/README.md b/README.md
index 8b6f3d8..21a8789 100644
--- a/README.md
+++ b/README.md
@@ -108,7 +108,7 @@ arg.MustParse(&args)
 
 ```shell
 $ ./example -h
-usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]] 
+usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]]
 
 positional arguments:
   input
@@ -148,6 +148,22 @@ fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 Fetching the following IDs from foo: [1 2 3]
 ```
 
+### Arguments that can be specified multiple times, mixed with positionals
+```go
+var args struct {
+    Commands  []string `arg:"-c,separate"`
+    Files     []string `arg:"-f,separate"`
+    Databases []string `arg:"positional"`
+}
+```
+
+```shell
+./example -c cmd1 db1 -f file1 db2 -c cmd2 -f file2 -f file3 db3 -c cmd3
+Commands: [cmd1 cmd2 cmd3]
+Files [file1 file2 file3]
+Databases [db1 db2 db3]
+```
+
 ### Custom validation
 ```go
 var args struct {