Improve markup processing

[jdkato]

• HTML: via golang.org/x/net/html.
• Markdown: via Blackfriday.
• reStructuredText: via rst2html.
• AsciiDoc: via AsciiDoctor.
• LaTeX: via Pandoc.

[jdkato]

I think the best strategy for reStructuredText, AsciiDoc and LaTeX is to rely on external programs (such as rst2html and Asciidoctor) to convert them into HTML. We can then use the net/html package to get what we need.

This obviously goes against the goal of being standalone, but it's the only realistic option for accurately targeting specific scopes (e.g., ignoring code spans and URLs or assigning certain rules to headings). I also think it's reasonable to assume that users of these formats will already have the required programs installed (for instance, rst2html comes with both docutils and Sphinx), and we should be able to get their location using exec.LookPath.

(For what it's worth, Hugo uses the same technique.)