virtual text support (#241)

* Start on virtual text support

* Working accept and clear

* Map keys. Rename functions as snake case

* Updates and fixes

- Add manual setting which disables automatic submission of completion
  request
- Implement cancel of in-flight request in clear function

* some README updates for config

* permit cmp to not be installed if not using cmp source

* More information on virtual text in README

* Better check for not setting key bindings

* Install highlight styles

* Implement status line

* update language

* Fix virtual text priority

* change "unknown filetype" notify to debug

* Use util.get_other_documents and util.get_newline instead of ported Vim implementation.

* Pass workspace_uri with completion doc, like Source:complete does

* some cleanup

* A bit more cleanup

* Convert from vim-style varargs to opts object

* Proper clear binding setup

* Configure accept fallback through setup instead of global

* Ignore buffers whose names are already URIs

This indicates special buffers created by some plugins like oil.nvim,
and the LS will return an error.

* Allow enabling and disabling virtual text by filetype

* Fix README typos

* Remove <c-[> default clear binding since that is also Esc

* Suggest setting `false` instead of empty string to disable key bindings