Reformat render_markdown for readability

Limit lines to 100 characters, and keep the name of the library we're
using at the callsite instead of only in the import.
Actually, move render_markdown to markdown.render

This makes it easier to permalink to the actual call site in our code
from the help text on the input prompt.
Link to markdown.py from help text

We're open source! We might as well make the most of it. :-)
Simplify module again

Don't need the hijinks now that it's in it's own file.
Revise help text links
Merge pull request #2736 from gratipay/specify-markdown-variant

document markdown variant inline