Add code snippet features to match features from sphinx literalinclude directive

[danroth27]

When using Sphinx to build documentation you can use the literalinclude directive to include code snippets from a separate file. DocFX flavored markdown supports something similar with code snippets, but it looks like it is missing a few features that we use heavily in the ASP.NET docs:

• Ability to specify multiple line number ranges and to skip lines:

.. literalinclude:: example.py
   :lines: 1,3,5-10,20-

• Options to display line numbers when the code is rendered and to emphasize specific lines when the code rendered:

.. literalinclude:: example.rb
   :language: ruby
   :emphasize-lines: 12,15-18
   :linenos:

• Ability to dedent the rendered code so that it is correctly aligned with the rest of the content instead of having extra leading whitespace:

.. literalinclude:: example.rb
   :language: ruby
   :dedent: 4
   :lines: 10-15

[superyyrrzz]

Support highlight lines in code snippet in 27e88f3