Be able to specify github domains

[owenlin0]

Just recently switched from Vim to Spacemacs, which includes this package for browsing Github. My company uses Github enterprise at work so it'd be nice if we could specify a Github domain like we can do for vim-fugitive. By setting a variable in the ~/.vimrc, you could tell Fugitive where to open Github links. Fugitive somehow figures out the right path (Github Enterprise vs. Github.com).

Usage in vim-fugitive:

1. Set let g:fugitive_github_domains = ['github.com', 'git.musta.ch']
2. :Gbrowse is now configured to properly look up from the list of Github domains.

https://github.com/tpope/vim-fugitive/blob/c00ebd75ac23f4080c0d0bf9453b16304a3fb316/plugin/fugitive.vim#L2351

[slackorama]

I took a stab at this with #22