mutt_ldap.py: Follow the the XDG Base Directory Specification

Standards are good, especially standards for keeping a plethora of
config and data files files organized.  Use PyXDG (an optional new
dependency) to handle all of the specification logic.

The default config file is now ~/.config/mutt-ldap.cfg and the default
cache file is now ~/.cache/mutt-ldap.json.  The contents of these
files has not changed.  Without the PyXDG dependency, these defaults
are hard-coded.  With the PyXDG dependency, the defaults shift
depending on $XDG_CONFIG_HOME, $XDG_CACHE_HOME, etc.