cache (new feature): allow use of cache directory outside of ${HOME}

[salewski]

The ads-github-cache program currently uses the following directory location unconditionally for the cached objects:

    ${HOME}/.ads-github-tools.d

It would be convenient, especially while doing development or even just experimenting, if there were a command line option to override the default location for a particular invocation of the program.

Bonus points for defining an environment variable that could be used for this purpose, as well. Maybe:

    ADS_GITHUB_TOOLS_CACHE_DIR

In the presence of both the environment variable and the command line option, the command line option would take precedence.