use neovim if command is present otherwise use vim

[onlurking]

this PR will make vimgolf use neovim as the default editor if is installed in the computer, the self function is multiplataform so it will run on Windows too.

[onlurking]

I don't know if using neovim as the default editor would be a sane choice, we maybe can discuss further this PR with some choices:

• use neovim if user pass a custom flag to vimgolf cli
• use neovim if it exists in PATH and vim doesn't

[filbranden]

If you prefer to use NeoVim for VimGolf, you can always set the environment variable:

export GOLFVIM=nvim

You can do so in your bash_profile, bashrc, zsh initialization script, etc.

Personally, I don't think using NeoVim by default if it's enabled is the best approach... But I can see ways in which we could make this easier to select (perhaps in the config file, selected through vimgolf setup, etc.) or we could ask the first time we launch, etc.

[danilobellini]

Many of the current solutions would not work in neovim, which makes me believe it should not be the default. Also, before enabling neovim, I think some changes to the .vimrc would be required as part of the "level playing field" requirement, including something like:

if has('nvim')
    unmap Y
endif