It has been noted in issue PyCQA#363 that match & match_dir are
unwieldy when attempting to match against full paths.  For
unexample if you have A.py in directories B & C and you only
want to run pydocstyle on one of them.

From my own experience trying to deploy pydocstyle against a
large legacy codebase it is unworkable as it would mean the
entire codebase being converted as a big bang change. A more
nuanced approach means the codebase can be converted gradually.

This commit adds a new option, match_path, to the config &
command lines which can be used to provide more nuanced
matching.  For example the following specification:

match_path = [AB]/[ab].py
             D/e.py

This defines two regexes.  If either match a given path,
relative to the directory specified, the file will be yielded
for comparison.  The is complimentary to match & match_dir and
the three can be used together.