Add support for 'keywords' key in .accomplishment files

As we want to implement search feature in the viewer for 0.4, the .accomplishment files would ideally consist of an additional key: 'keywords'. It would be a list of possible search terms for this accomplishment. For example, for ubuntu-community/community-council-member it might look as follows:

  keywords = cc, governance, comunity

This way it would satisfy almost all search queries that one might type in the search bar, which are not included in accomplishment's title. The above example also includes a common typo.

Such mechanism would cause more relevant search results, and is much faster then processing all these long descriptions (which may happen not to contain some synonyms - like 'askubuntu' in Ask Ubuntu accomplishments).

Ideally, that field would be translatable.