Assume initialisms aren't acronyms by default

[glacials]

Adds broader support for initialisms. Currently, all initialisms are assumed to be pronounceable (e.g. NAFTA) so the a/an decision uses the same logic as lowercase words, with a list of overrides for non-pronounceable initialisms (e.g. NPC).

But overall it feels like the majority of all-uppercase words will not be pronounceable, so IMO the a/an decision should default to how the first letter is pronounced (e.g. "eff" for F), with a list of overrides for pronounceable all-caps words.

(I don't have any data to support that feeling though, so feel free to reject that part and I'll just put in overrides for the words that were causing me trouble 😃 )

[robinaugh]

This scratches an itch we have with this library as well (for acronyms like HOA)! Would love to see this go in.