Auto-anchor attribute regexes, document _validTags
FIX: Use lang- prefixes for HLJS; add nohighlight for ...

text; use a proper whiteListTag() call.

Old posts are not affected - no global rebake is necessary.
Clean up validateAttribute() function

Remove code duplication
FEATURE: Allow explicit rel="nofollow" on links
Fix JSHint
Fix data-* behavior (failure to understand own docs)
Update Markdown tests
Merge pull request #2848 from riking/whitelist-api

Improve Markdown.whiteListTag, code dialect