Can now seed identifier database with keywords

By toggling the g:ycm_seed_identifiers_with_syntax option, the user can now tell
YCM to seed the identifier database with the language's keywords.

This is off by default because it can be noisy. Since the identifier completer
collects identifiers from buffers as the user visits them, the keywords that the
user cares about will already be in the database, regardless of the state of the
new option. So the only keywords added will be the ones the user is not using.

Meh. But people want it so there.

Fixes #142.