Feature: Do expansion on all path options in configuration

[hetmankp]

Previously issues #6508 and #7259 added home directory and environment variable expansion to the cache_dir option in the configuration file. It might make sense to the the same for all the other options also referring to paths, these would include: files, mypy_path, custom_typeshed_dir and python_executable.

Some review may be necessary to determine if this is not appropriate for any of these options.