Avoid duplicated output using highlight tags

While using Rouge and an `highlight` tag, the output was duplicated
since the `output` variable in the Liquid tag definition was equal to
the highlighter's prefix value and the `<<` method changes its receiver.

Therefore, we should simply define an empty string and append the prefix
if it is present.
Collections' "written" attribute has been changed to "output".

This is a follow-up to 068d921ef649501ffc8cf57f2a3b124c7378a9c5.
Update history to reflect merge of #2264 [ci skip]
Refactor 'jekyll serve' command.
Update jekyll-sass-converter to the latest RC

To see a detailed history of the changes, please visit https://github.com/jekyll/jekyll-sass-converter/compare/v1.0.0.rc3...v1.0.0.rc4

Notable change is that you can now set the style of the outputted Sass.
Ref: https://github.com/jekyll/jekyll-sass-converter/issues/3
Merge pull request #2269 from jekyll/fix-up-serve
Update history to reflect merge of #2269 [ci skip]
Refactor highlighting

Extract some of the common logic from the renderers.
Context is not used within any of the renders so no need to pass it
Revert back to an intialize more similar to the original
Merge pull request #2154 from jpiasetz/refactor-highlighter
Update history to reflect merge of #2154 [ci skip]
Update util functions

Cribbed from http://api.rubyonrails.org/classes/Hash.html#method-i-symbolize_keys
Merge pull request #2273 from jpiasetz/copy-rails-functions
Update history to reflect merge of #2273 [ci skip]