Add Comparison

[rcdilorenzo]

It would be helpful if using this library for models that need to be generated within some range to be able to compare the repeats. Below is an example of this sort of usage:

import Repeatex, only: [parse: 1, compare_interval: 2]

assert compare_interval(parse("every day"), parse("every other day")) == :lt
assert compare_interval(parse("every day"), parse("every day")) == :eq
assert compare_interval(parse("mon-sat every week"), parse("every day")) == :gt
assert compare_interval(parse("monday every week"), parse("mon-wed every week")) == :eq

This would need to be delegated off to each corresponding model type and added to the appropriate protocol. It might also require some sort of general conversion from the type to the number of days (or range?) for a interval.