Context aware completion

[Deraen]

https://github.com/tpope/vim-fireplace/blob/master/plugin/fireplace.vim#L113-L142

Fireplace function is private so we can't call it. Additionally it doesn't work correctly with deoplete.

With omnicomplete the complete-str is removed from buffer when completion is triggered. With deoplete this doesn't happen and the complete-str should be removed manually from the result.

Would really help if we could write unit tests.