Add NeoVim support and extra functionalities

[esseeffeenne]

1. Wraps a few methods and v:t_job around a has('nvim') clause to make it work on latest neovim
2. Define a clearer default value if not websocket PID is found
3. Wraps wwws#conn#Send(...) around a function parametrized against a motion, so it's effortless to create new functions
4. I've added wwws#buffer#SendLine(), wwws#buffer#SendHeadless() and their Delete counterpart, which clears the buffer and stores it inside the unnamed register
5. Add extra mappings for the new functions by default.
6. All files are formatted with vim indent for consistency.

Tested:
NeoVim nightly v0.12
Vim latest v9.1