added better stem changing verbs support

still need to add some more verbs to the list, also other
irregularities like "go" verbs and "zco" verbs should be added into the
main code (there's a list of zco verbs right now),  verb lists from
http://www.intro2spanish.com/verbs/listas/master-ar-zar.htm, figure out
why the subjunctive conjugation isn't taking the stem changes from the
present indicative conjugation