Decouple CoffeeScript and Sass gems.

Strong coupling requires a huge investment by users in terms of
troubleshooting and other packages (e.g. NodeJS or therubyracer)
Running `gem install <blah>` isn't a significant cost, especially
given that the user ran 'gem install jekyll' just fine. If this
process is automated, adding in these gems and their dependencies
is easy and can be automated too.

To install either of these gems, run `gem install <gem_name>`
or add them to your site's Gemfile (if exists) by adding
`gem <gem_name>` to your site's Gemfile, then run `bundle install`.
They will be automatically required – no work beyond that!

Jekyll shall not be bloatware.

- [x] Decouple
- [x] Write installation instructions in docs

Fixes #2335.