A Bunch Tell is now Musing Studio.

This fixes build issues we're having due to the SSL certificate
on git.mills.io. It also updates to the latest version of the
go-gopher library.

Fixes #496

Previously, if running an instance on e.g. http://localhost:8080, the port
would show up in the Gopher links and potentially cause rendering to fail.
This fixes that.

This will derive the host from the configured host by stripping the protocol from the URI

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