Remove trailing spaces from output lines

Add index option to RunAgain, update docs

Added partial support for Neovim

Neovim's job control is implemented differently from that of Vim 8.0's.
You can still run and manage jobs, but streaming stdout to buffers and sending
keys to a running job is not supported.

Disabled command editing for split mode

Opening a split while the BufWinLeave autocmd is running causes issues.
Tried several workarounds to no avail, so the most workable solution is
to disable it for split/vsplit (not that it'll be used that much anyway).

First release