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
   - `[][]` internal links
   - `%` and `#` sigils

Fixes: #51
3 jobs for check-command in 1 minute and 50 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Check
passed flake8 #1330898

00:01:46

failed mypy #1330899
allowed to fail

00:01:50

passed reuse #1330900

00:00:57

 
Name Stage Failure
failed
mypy Check
gidocgen/gir/parser.py:982: error: Argument "get_type" to "GType" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1004: error: Argument "name" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1004: error: Argument "symbol_prefix" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1004: error: Argument "ctype" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1006: error: Argument 1 to "set_version" of "GIRElement" has incompatible type "Optional[str]"; expected "str"
gidocgen/utils.py:808: error: Need type annotation for 'found_programs' (hint: "found_programs: Dict[<type>, <type>] = ...")
Found 206 errors in 4 files (checked 17 source files)
Cleaning up file based variables
ERROR: Job failed: exit code 1