Reduce clutter in gembased theme

[peterdesmet]

petridish can now be installed as a gem (see #84). I used the default spec.files

https://github.com/peterdesmet/petridish/blob/d656bf7541266561fcdd3ee69798d038472bf90b/jekyll-theme-petridish.gemspec#L13

But that results in quite a lot of files, resulting a fresh site that looks like this:

What we should have:

• [x] assets: keep, required for fonts, font-awesome, javascript, etc. Is all neatly wrapped in assets/theme subdir
• [ ] _data: maybe it's not necessary to already have a header, footer?
• [x] _layouts: keep
• [x] _includes: keep
• [x] _sass: keep
• [ ] LICENSE: keep?
• [ ] README: keep>
• [ ] _config.yml: likely remove, I don't think fallbacks are useful here

This is what minima has:

https://github.com/pages-themes/minimal/blob/master/jekyll-theme-minimal.gemspec#LL13C19-L13C50