Implement the diagnostics pull model with the LSP Neovim client. We
must handle messages a little different and tweak client capabilities
for pull diagnostics to work through the Neovim client.
Ensure that basic ALE functions `ale.var`, `ale.escape`, and `ale.env`
are available in Lua. Cover all Lua code so far with busted tests,
fixing bugs where ALE variables can be set with Boolean values instead
of numbers. Document all functionality so far.
