Instead of just using Flake8 and pylint to keep Flake8 clean, let's also
use black to make it less manual for clean-up.

Also fills out the hg install hook docstring.

This would close GitLab issue #335.

Apparently there are some cases where the git hook will not find files
to run the checks against (e.g., when amending a commit message). In
those cases, it's best not to attempt to run any checks and to allow the
hook to exit successfully.

Closes #303

Instead of displaying the path to the temporary directory, as we always
have, it's been requested that we instead display the path to the file
that was copied to the temporary directory.

Closes #244

Previously we added support so users can do:

    $ flake8 bin/executable

But this broke the fact that git hooks shouldn't check things like
reStructuredText doc files. This commit restores that functionality but
will cause bin/executable to be ignored in the git hook. This seems fair
since folks can amend their filename patterns to include it explicitly.

Closes #268

When running the git hook, even though the appropriate config file is
found and processed, the exclude patterns and paths were not being
corrected to account for the temporary directory that we copy the files
into.

Related-to #223

Look for configuration files in the current directory while checking the
files in the temporary directory.

Closes #210
Closes #223