Since LUA is single threaded I reached a limit to performance
optimization, both 'git_icons' and 'file_icons' require string
matching and manipulations which eventually hurt performance
when running on large amount of files.
In order to solve that this commit introduces the option to spawn
commands and process the entries in a separate neovim process which
prints to stdio as if it was a regular shell command. This speeds up
things significantly and also makes the UI super responsive as if fzf
was run in the shell. This required a few lua hacks to be able to load
nvim-web-devicons in a '--headless --clean' instance and sharing the
user configuration through the RPC interface from the running instance.
This is enabled by default for 'files' and 'grep' providers and can also
be enabled for 'git.files' if required, control using the 'multiprocess'
option.
bhagwan