Highlight GitHub style fenced code blocks

One needs to opt-in to this by specifying which file types should be
highlighted:

    let g:markdown_github_languages = ['ruby', 'erb=eruby']

This may not work for all file types.

Closes #17.
Fix ```language with more junk on the same line
Really fix options after ```language
A more liberal markdownValid
Allow automatic links in headings
Require space after ordered list marker
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
Merge branch 'master' into highlight-code