add interface for telling MathCAT what (all of) the user prefs are

For some systems such as NVDA, they have their own preference files. Rather than forcing everyone to use MathCAT's preferences (and not being integrated into uninstall, etc), provide a means to pass in a YAML string (JSON is a subset of YAML, so JSON works also) that are MathCAT's preferences.

MathCAT never writes preferences (the dialog does), so no need to return preferences.