Support indented fenced code blocks
Fix highlighting HTML entities at start of line
Support blank lines in block quotes
Clear `main_syntax` if we set it.

This prevents conflicts with other syntaxes that use `main_syntax`, like
vim-javascript.
Highlight italicized regions within bold regions
Merge pull request #38 from hjdivad/dont-stomp-main-syntax

Clear `main_syntax` if we set it.
Merge pull request #41 from cemerick/master

Highlight italicized regions within bold regions
Allow spaces after ```

With a fenced code block with a language, it's legal to put spaces
between the fence and the language name.
Merge pull request #45 from AndrewRadev/fix-fenced-code-block-pattern

Allow spaces after ```