Allow to add faked modules to patcher

[mrbean-bremen]

Sometimes it makes sense to add an own fake module for a module name.
Possible use cases:

• a part of a module often imported in its own namespace like from pathlib import Path (see When 'Path' from 'pathlib' is imported, it isn't faked. #344)
• a module implements Python functionality for earlier versions (like scandir, see Pyfakefs doesn't work with Python 2.6+ backport of os.scandir #332)
• a module is using the file system via the C API and shall be replaced by a pure python module in tests only (see for example Cannot use pyfakefs together with lxml #180)

Possible API: add an argument additional_fake_modules to Patcher containing a list of dictionaries. Each entry holds the module name with the respective fake class.