Themes: support for a local _themes folder

• This is a feature request.

As I understand Jekyll now, it reads layout, includes, etc. from 2 places:

1. The actual site (cwd)
2. The theme, if one is set in _config.yml and the gem exists

Both work, both have challenges. The challenge with the first is mixing content and style, and updates are messy (which is why gem-based themes were created). The challenge with gem-based themes is that you may be running in an environment that restricts gems (notably Github Pages). bundle install --deployment would help here due to strict whitelisting.

Would the Jekyll team consider a third: local themes? I am looking at a structure like:

- site
   |- _posts
   |- assets
   |- _config.yml
   ...
   |- _themes/
        |- minima/
        |- custom1/
        ...

All that a current Gem-packaged theme does is read the files first from the Gem dir and then override with local. The above would mean just reading Gem, then local under _themes/, then whatever is in the local site.

Happy to do a PR if idea is accepted in principle (and if someone points me to where the includes of themes happen).

(apologies if idea already has been discussed; couldn't find on issues, PRs or talk)