Migrate to use the LSP

[fredemmott]

Ideally use hh_client --version --json and use legacy approach if api_version < 5.

The neovim LanguageClient plugin also works happily on vim 8 - I'm using it with MacOS High Sierra's default vim with fredemmott/dotfiles@55054f2

In that setup, I'm basically just using this plugin for syntax highlighting, LSP for everything else.