'defnoun' doesn't seem to work when the word is a part of a string passed

Originally reported by @aMiss-aWry in #161:

> I can fix the specific words with defnoun('jeans', 'jeans'), which corrects the result for an isolated string 'jeans' -> 'jeans'.
>
> However, if I try to run plural_noun on 'blue jeans' instead of 'jeans' it then breaks again, returning 'blue jeanss'. I understand this is because 'blue jeans' appears to be a different string from 'jeans', but perhaps it should check the rules for the part of the string it is altering?

Start by reproducing the issue through plural_noun('blue jeans') after defining 'jeans' with defnoun. Trace how defnoun rules are matched within longer strings, then add coverage so the configured form is preserved when the word appears as part of a string.

Written by the indexing model from the issue text.