Render code as a table

Putting this here to get some feedback from you.

This is a small initial part of a number of features I want to propose.

The main idea is to have a custom code render function that gives us more flexibility over the code presentation. Based on this, we could have inline comments right in the code (e.g. GitHub, Phabricator, etc.)

Fixes #1272 but breaks select-and-copy (because line numbers are included into copied text)