Adding option to follow symlinks when generating API

[leakec]

• Adding autoapi_follow_symlinks, which can optionally be enabled to follow symlinked directories when os.walking to generate the API docs.
• Added documentation for autoapi_follow_symlinks.
• Added unit tests for autoapi_follow_symlinks.
• Fixed mypy error in _mapper.py. This was broken before I made any changes, but fixed it so all the unit tests are passing. I just handled the None case by explicitly throwing an error if the match returns None. This would cause an error in the next line anyway when match.groups() was called, since None doens't contain groups().