Work around users overriding ftplugin/html.vim
Flag internal underscores in headings
Ignore options after ```filetype

Closes #22.
Improve tab handling

Closes #18.
Strip r and o out of format options

Closes #25.
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
Set grip as makeprg

Set grip as makeprg for any markdown file. This will launch an http
server on localhost:5000 with the current buffer rendered into HTML.