When a Liquid Exception is caught, show the full path (rel to site source)
Docs: Explain what goes into the redcarpet: extensions: array

- Document the two secret-sauce extensions that Jekyll adds, which don't seem to
  be mentioned elsewhere.
- Link to the Redcarpet documentation for other extensions.
- Mention the three or so most common extensions.
- Mention that Redcarpet 2.2.2 lacks several extensions that were added later.
Update curly apostrophes
Merge pull request #1419 from penibelst/typo-apostrophe

Update curly apostrophes in documentation
Update history to reflect merge of #1419
Merge pull request #1418 from nfagerlund/docs_redcarpet

Docs: Explain what goes into the redcarpet: extensions: array
Update history to reflect merge of #1418
Merge pull request #1415 from mojombo/full-path-output-for-liquid-exception

When a Liquid Exception is caught, show the full path (rel to site source)
Update history to reflect merge of #1415