Dinodex

with help from @jvanderz22 and @jillalynch
i wasn't sure about this one either. i was trying to solve the problem of wanting to match period: "cretaceous" to "late cretaceous" and "early cretaceous", while at the same time *not* wanting continent: "america" to match both "north america" and "south america". 

the other solution i was contemplating was writing separate `matches_period?` / `matches_description?` methods in the same style of `matches_weight_in_lbs?` and `matches_diet?` -- except the implementation of both would be exactly the same, hence trying to collapse them into one method.