altosuilib: Add google maps API key, configured with -with-google-key

This places the actual key outside of the repository, allowing the
user to configure the name of the file containing the key. By default,
this pulls the key from $HOME/altusmetrumllc/google-maps-api-key.

With the key present, there are no longer any rate limits to loading
map data.

Signed-off-by: Keith Packard <keithp@keithp.com>