Wrote new method `choices_left` in commands package module, which
will play a part in autocompletion feature. The method takes an incomplete
input, like the `readline` autocomplete functionality would expect it,
and identifies possible appendices that can be appended without making
the input invalid.

TODO: It works so far, except apparently in case an illegal argument value
is with what the input fragment ends. Disregarding the invalidity of
of the last value, the method still returns valid suggestions for
succeeding keywords/arguments. But his might just be fine for now.
simple test script for parsing/registration/pre-autocomplete debugging,