Completed(?) work on selection of verb forms

* input.c
** Added function to check input and set flags accordingly
* parts_of_speech.c
** Added adjectives as a part of speech
** Added constants for "cardinality," non-finite forms
* verbs.c
** Added an enum for magic numbers as flags
** Implemented selection for all currently-present verb forms
* main.c
** Nothing significant, just changes for testing