Allow user to specify build tool

[aidan-gallagher]

I believe most Debian packages use format 3.0 (quilt).

This format requires an upstream tarball. I believe most people use git-buildpackage to automate the creation of the upstream tarball.

Debpic should let the user invoke gbp buildpackage instead of dpkg-buildpackage within the container.

This can kind of be done with the current setup by doing this - however it is clunky.

debpic --extra-pkg git-buildpackage "gbp buildpackage"