v0.2.0: combine pluralization and singularization rules (#30)

* combine pluralization and singularization rules

Combines the rules for pluralization and singularization into a single
set for improved consistency and maintenance.
Suffix rules are now ordered with most specific matches (longest suffixes)
matched before shorter suffixes.

Also fixed pluralization of "you", should be "you" instead of "yous".
And added some extra cases for better handling of some words ending with
"us".

* remove unused rule

The rule for "appendix" suffix is not used because it's already
in the list of full words.

* alphabetize single to plural word map