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

[jaraco]

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?

[jaraco]

These compound (multi-word) phrases are a more complex challenge in this library. The library does attempt in some cases to handle such words, but probably can't do so in general. Consider for example "a multitude of factors" or "cul de sac", which when pluralized becomes "multitudes of factors" and "culs de sac". I welcome someone to try to solve this issue either generally or specifically for "(anything) jeans".