Add support for explicitly interrogating .pyi files.

[psobot]

In some cases, it may be useful to interrogate .pyi files (i.e.: when checking docstring coverage of native Pybind11 modules). Interrogate can parse these files, but refuses to due to a file extension check.

This PR adds a quick patch to work around the issue, while also fixing a typo in an error message. Note that this will not automatically pick up .pyi files in directory trees; .pyi files will only be parsed when passed explicitly to interrogate as filenames.