Add a message and error out when Pygments returns nil.
Went with @stomar's suggestion
Add jekyll_figure plugin
Merge pull request #2158 from lmullen/patch-1
Update history to reflect merge of #2158 [ci skip]
Merge pull request #2148 from jekyll/error-when-pygments-errors
Update history to reflect merge of #2148 [ci skip]
Refactor highlighting

Extract some of the common logic from the renderers.