Included appdirs in project

The prior attempt to resolve #420 and #431 relied on `appdirs` to
determine the appropriate directories into which to install files.
Previously, `setup.py` dynamically attempted to install `appdirs` via
`pip` during installation if necessary.

This attempt to failed on multiple platforms, however, due to
backwards-incompatible `pip` interfaces.

As a workaround, I have now directly included `appdirs` (a small module)
within `cheat` itself. This approach is explicitly supported per the
`appdirs` documentation:

https://pypi.org/project/appdirs/

ci/lint.sh

@@ -7,4 +7,4 @@ APPROOT=`realpath "$SCRIPTPATH/.."`
 
 flake8 $APPROOT/setup.py
 flake8 $APPROOT/bin/cheat
-flake8 $APPROOT/cheat/*.py
+flake8 $APPROOT/cheat/*.py --exclude=appdirs.py