Implement URL conflict checking in `jekyll-doctor`.
Nuke comment.
Call ruby's 'abort' if the site is unhealthy
When a Liquid Exception is caught, show the full path (rel to site source)
Merge pull request #1411 from penibelst/master

Correct internal links
Update history to reflect merge of #1411
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
Fix link to contributing page in README. Fixes #1424.
Update history to reflect fixing of #1424.
Replace defunked mailing list link with info on IRC room.
Use Doctor#healthy? and ensure _all_ items are checked before process quits.
Merge pull request #1389 from mojombo/doctor-conflicting-permalinks

Implement URL conflict checking in `jekyll-doctor`.
Update history to reflect merge of #1389