Reduce clutter in gembased theme

[peterdesmet]

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

petridish/jekyll-theme-petridish.gemspec

Line 13 in d656bf7

spec.files = `git ls-files -z`.split("\x0").select { |f| f.match(%r!^(assets|_data|_layouts|_includes|_sass|LICENSE|README|_config\.yml)!i) }

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

What we should have:

• 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?
• _layouts: keep
• _includes: keep
• _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