Add an option to raise a warning when some modules are not found

[joubs]

As stated in the documentation: "Every module you pass in to pdocs must be installed or otherwise available on your PYTHON_PATH".

I would be really interested to add an option to skip unfound modules and raise a warning, in order to use pdocs in one of my projects. For some reason I rely on some third party modules I cannot install on every machine, but I would still be interested to generate the documentation from the CI.

Does it sound like an acceptable addition? I am happy to draft a PR if so.