Add Make-compatible depfile generation (Fixes/Supersedes #41)

[ijatinydv]

This PR revives and finishes up the depfile generation work originally started in #41.

I built on the original implementation with two main tweaks to make it robust for programmatic use:

1. Integrated execution: process_file() now automatically handles writing the depfile if the --depfile option is passed, mirroring standard compiler behavior (like GCC's -MMD).
2. Stateless dependency tracking: Fixed a bug where reusing a Parser or Fypp instance across multiple files would leak stale dependencies from previous runs into the new depfile.

Also added a few tests to verify the Make-escaping logic (spaces, #, $). Let me know if anything needs tweaking!