Teach scss-lint --show-formatters

Show the installed formatters conveniently on the commandline.

    $ scss-lint --show-formatters
    Installed formatters:
     - Config
     - Default
     - Files
     - XML

Change-Id: Ic5aee057b7eea2ec847663b5d4e4c190e790a665
Reviewed-on: http://gerrit.causes.com/37742
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>