v0.1.0: Breaking changes for public release

We're almost ready to talk about cage in public!

Breaking changes (we rushed a bunch of these in under the wire before
the public release to minimize breakage later):

- Rename `repo` → `source`, and `repo list` → `source ls`.
- Rename `config/libraries.yml` to `config/sources.yml` and change
  format.
- Rename `override` → `target` everywhere: `--target`, `pods/targets`,
  `enable_in_target`, etc.

Other improvements:

- Add `source mount` and `source unmount` commands.
- Automatically generate `config/secrets.yml` for new projects and
  demonstrate good secret handling.
- Allow source code for services to be part of the main repository.

Bug fixes:

- Fix bug where we mounted local library source trees even if they
  didn't exist.