Add description metadata when available.

This should fix the problem we have in Google search results. Currently
we show up as

**H2GIS • Spatial H2**
www.h2gis.org/
wget http://tinyurl.com/h2gis-zip. -O h2gis.zip. ~ $ unzip h2gis.zip. ~
$ cd h2gis-standalone. ~/h2gis-standalone $ ./run.sh. # => Click Connect
in the web interface.

Hopefully after this commit, we will show up as

**H2GIS • Spatial H2**
www.h2gis.org/
A spatial extension of the H2 database. Compliant, Solid, Tested.

Note: Function descriptions from the YAML front-matter should also show
up in search results.