Add support for installing Neovim

[blueyed]

Might be as easy as the following for nightlies (works for Travis):

  eval "$(curl -Ss https://raw.githubusercontent.com/neovim/bot-ci/master/scripts/travis-setup.sh) nightly-x64"

https://github.com/neovim/bot-ci/blob/master/scripts/travis-setup.sh
https://github.com/neovim/bot-ci#nightly-builds

Support for specific versions might me nice though, too - but not really required at the moment.

[tweekmonster]

Oh cool, that'll be useful. This was something I wanted to look into doing.

[blueyed]

Can you provide some input of how it should be done?
Should there be a install_nvim.sh script?

[tweekmonster]

Sorry this fell off my todo list. I haven't actually tried it yet, but it should just need to be installed in a place where run_vim.sh expects it.

Should there be a install_nvim.sh script?

Yeah, I think that would keep it simple. XDG_ variables should probably be set as well.

My thoughts left off on trying to get it to actually build the release versions in addition to nightly, but it wasn't working without using pre-built packages for libvterm and libtermkey from the testing channel(?).

I was also thinking about removing the requirement of sourcing /rtp.vim and having a standard vimrc script that loads a known script name from the volume instead.

Paging @mhinz in case he made some breakthrough with building Neovim in Alpine docker.

[blueyed]

I've created #8 for the better-vimrc-rtp.vim-handling.

[w0rp]

I'm looking forward to this. Thank you for writing vim-testbed. It has been very useful for testing a Vim plugin I have been working on.