[WIP] contrib: localvimrc

Is anyone at all using https://github.com/neovim/neovim/tree/master/contrib/vim-addon-local-vimrc ?
You *must not* use `exe set`. It is too hard to get proper escaping here. If you want to modify this setting use `let &path .= (empty(&path)?'':',').escape(s:srcroot, '\,').'/**'`.