Add forgotten file, src/common/github.lisp
For now, refrain from using the extension fullname everywhere.

The code base isn't ready to deal with fullnames only internally. What
needs to happen is some code that knows how to parse an extension name
and always return the fullname, but we don't have that yet.
Add the `fetch` command.

This new command allows to use pginstall at the command line as a
download client to fetch an archive:

  ./build/bin/pginstall fetch prefix 9.4devel /tmp