Add list command

[tcnksm]

To check available framework,

$ gcli list
+-----------------+---------+-----------------------------------------------------+
|      NAME       | COMMAND |                         URL                         |
+-----------------+---------+-----------------------------------------------------+
| mitchellh_cli   | *       | https://github.com/mitchellh/cli                    |
| codegangsta_cli | *       | https://github.com/codegangsta/cli                  |
| go_cmd          | *       | https://github.com/golang/go/tree/master/src/cmd/go |
| flag            |         | https://golang.org/pkg/flag/                        |
+-----------------+---------+-----------------------------------------------------+
COMMAND(*) means you can create command pattern CLI with that framework (you can use --command flag)