Allow building of multiple packages with one command

[tombettany]

Something like dr build kano-blocks make-pong to build both Kano-Blocks and Make-Pong.

What needs to be done

1. Investigate how could we extend the interface of dr build to be able to build several packages at once. This could be tricky, because of the --branch and --push options that might differ for different packages.
2. Implement those changes.