[WIP] Fix #2690 - Allow extra args to pass directly to Neovim

[bryphe]

This is a fix proposed by @CrossR for #2690 - we can pass any additional arguments after a -- directly to Neovim. For example, oni some-file.js -- -S my-session.vim

Still needs to be tested / validated.

[justinmk]

For what it's worth, my thoughts on this are at equalsraf/neovim-qt#449 .

Unless the UI plans to introduce a bunch of CLI options that potentially conflict with nvim options, it would be better to just forward all unrecognized options to nvim, rather than using this -- workaround.