Add tag to kata

[yvoyer]

Add possibility to tag a kata with a list of string values. Tags are configured in the kata configuration.

Scenario: Show the list of tags
Given The kata 'kata 1' is tagged with tags 'Tag1'
And The kata 'kata 2' is tagged with tags 'Tag1, Tag2, Tag3'
And The kata 'kata 3' is tagged with tags 'Tag2'
When I execute the 'show --tags' command
Then I should see 
"""
Available tags:

Tag1
Tag2
Tag3

Add --tags option to show command (depends on #29).

--tags: This option will list all tags from all kata
--tag={tag}: lists all kata linked to tag

Scenario: Show the katas tagged with specific tag.
Given The kata 'kata 1' is tagged with tags 'Tag1'
And The kata 'kata 2' is tagged with tags 'Tag1, Tag2, Tag3'
And The kata 'kata 3' is tagged with tags 'Tag2'
When I execute the 'show --tag Tag1' command
Then I should see 
"""
Kata grouped by tags

[Tag1]
  Kata 1
  Kata 2

[yvoyer]

@octopixelle I think this could be a reasonable feature for you. Are you up to take it?