Add CFEngine 3 Blueprint frontend

This frontend supports files (no templates) and packages only right now.  It goes together with https://github.com/cfengine/design-center/pull/299 which implements the CFEngine::Blueprint library to handle the generated calls.

Services and sources are TODO but not essential; packages and files are our top priority right now and we will tackle services and sources next.

The frontend generates a Design Center sketch, which is a self-contained installable package indexed by `sketch.json`.  All the files are packaged in the files/ subdirectory so installing the sketch will drop a copy of all the files in a staging area; then activating and running the sketch will actually install the files in their final destination.

Please give feedback on the code structure (Python newbie here) and anything else I may have missed.  Thank you.