[Features] Vertical indentation lines (Tracker #61079312)

Show vertical "swim lines" so users more easily track how far they're indented.

Notes:
* Does not show lines within sections that are markdown, JSON, or XML (though it does show lines to the left of these sections corresponding to the RAML they're nested under)
* Hard-coded for 2 spaces of indentation
* Shows vertical lines for white-space–only lines
* Works correctly for lines that wrap

For https://www.pivotaltracker.com/story/show/61079312