add code completion

maybe one of these:

Compilation database (as used by clang-tidy and others) is a very very big plus!

We may need to add some symlink mechanism so that the db is found when it resides in the build dir.

https://github.com/Shougo/deoplete.nvim

https://valloric.github.io/YouCompleteMe/ (I read it's too complicated to config, segfaults, I also read it supports CLANG compilation database which is used by C Template, important!)

https://github.com/maralla/completor.vim

https://github.com/Rip-Rip/clang_complete

https://github.com/cquery-project/cquery