Add YouCompleteMe plugin for vim

* It requires some extra installation steps, for instance initializing
it's submodules and running `./install.sh --clang-completer`.
* I might have to execute `:python` with
`execfile("activate_this.py", dict(__file__="active_this.py"))` (path to
`activate_this.py` must be present, like `env/bin/activate_this.py`.