Add a "check" command
We want to catch undocumented API before we merge it. The introspection scanner can fill in the defaults, and this might not cause warnings; but the documentation is another matter entirely.
Right now, we check only for missing documentation. In the future, we should add more checks, like:
- property and signal annotations
- invalid links
- legacy gtk-doc content
|[ ... ]|code blocks
Fixes: #51 (closed)