Support for keyword based syntax highlighting (much faster)

See the following issues on GitHub:
 - https://github.com/xolox/vim-easytags/issues/68
 - https://github.com/xolox/vim-easytags/pull/80

Please note that right now this 'feature' is not integrated with
the "accelerated Python syntax highlighting" feature, because I'm
considering ripping that out and replacing it with a *fast* Vim
script implementation (if I can build one :-).