Include specific lines from code snippets

[ryangyoung]

It would be useful to be able to include only specific lines of a given code snippet rather than the entire file.

Here's an example of Sphinx's implementation of this: http://www.sphinx-doc.org/en/stable/markup/code.html#includes

[mDuo13]

Made a working proof of this here: https://github.com/ripple/dactyl/compare/selective_include