New issue with mailto links

Links containing an embedded '@' should not create an email link.
minor performance optimizations
more useful to_s output for blocks
minor syntax changes, alias header? to has_header? on Document
resolves #863 allow doctype to be set in AsciiDoc table cell
Set background color on toc2 to cover scrolling content
Merge pull request #860 from @neher
Merge pull request #864 from mojavelinux/issue-863

resolves #863 allow doctype to be set in AsciiDoc table cell
resolves #866 don't match e-mail address inside URL
Merge pull request #849 from @LightGuard
optimize several loops (remove use of inject)
interpolation is faster than dup for string
use nowrap option of Pygments to simplify post-processing
minor syntax cleanups
minor syntax cleanups
more efficient way to check for Extensions class
optimizations and syntax cleanups
remove ERB from HTML5 document template
minor cleanups
minor optimization