Support for C/C++ semantic highlighting.

[davits]

For testing purposes only.

Continuation of pull request ycm-core/ycmd#291

For now, each time when new tokens are received I'm cleaning old matches and adding new ones. This is a quick/dirty solution.
Vim doesn't have effective mechanism for semantic highlighting. The matchaddpos() function I'm using has several "issues":

1. Added matches are staying at their absolute positions, and when user types, it messes up all the highlighting. One should wait for reparse to finish to normalize tokens. This may be frustrating...
2. Vim stores matches in a linked list, so adding/managing all the tokens at once may slow things down in the big files with lots of tokens. (Though I haven't tested this scenario yet)

[Valloric]

The performance aspect of Vim handling semantic highlighting is one of the main reasons why I don't think we should tackle it. I'm not convinced such a feature could be added in any performant way in Vim, purely because of Vim's misdesigns.

@puremourning @vheon @micbou

[davits]

Closing this PR to split it into smaller pieces.