Add Gopher support

This adds gopher support to WriteFreely -- both single- and multi-user
instances. It is off by default, but can be enabled with the new
gopher_port config value in the [server] section.

When enabled, multi-user instances will show all public blogs at
gopher://[host]:[gopher_port]/ -- otherwise, blogs are accessible at
gopher://[host]:[gopher_port]/[blog]/

This is just a proof of concept for now. We still need to handle some
edge cases and different configurations, like private instances.

Ref T559