Allow Force Enabling Emojis

[AGhost-7]

I'm using Neovim from a docker container and emojis aren't showing in my omnicomplete dropdown.

Looked at the code and it seems like the autodetection doesn't handle linux.
https://github.com/rhysd/github-complete.vim/blob/master/autoload/github_complete/emoji/data.vim#L898

I don't think there's really any way to figure out the host OS' fonts for the terminal, so if I can suggest anything it would be to allow users to force enable/disable the emojis from being displayed in the completion dropdown. Something like:

let g:github_complete#emoji#data#force_available = 1

[SuperSandro2000]

Great that you wrote a patch yourself AGhost-7@63497a5.

I really appreciate it! Thank you so much!

The only difference is that you named the option let g:github_complete_emoji_force_available = 1.