Add env variables CALCURSE_CALDAV_DATA_DIR and CALCURSE_CALDAV_CONFIG_DIR

[josephmturner]

Allows users to set arbitrary locations for caldav config and data, without the calcurse-caldav script creating unused caldav directories inside $XDG_DATA_HOME/calcurse and $XDG_CONFIG_HOME/calcurse.

Addresses #393

[lfos]

Thanks for your contribution, Joseph! If I'm not mistaken, all paths can already be configured using command-line arguments (--config, --hookdir, ...) is there anything that's not covered by them?

You're correct that using them will still lead to standard directories being generated to check_dir() calls. I think we should move the check_dir() calls to a later point in the script, after command line arguments have been parsed. Does that make sense?