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

[NSoiffer]

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.