Make it possible to write SQL function remarks in Markdown

I'm a big fan of Markdown. And I'm tired of trying to write function remarks in
html. So I made this little patch to be able to write function remarks in
Markdown and have them:

1. Rendered in Markdown in the SQL console (human-readable)
1. Rendered in HTML in the tooltips (pretty)

This adds a dependency on [Markdown4J](https://github.com/jdcasey/markdown4j) (~62K).

Slightly related to #643.