Increase LRU cache size to prevent thrashing, invalidate on "learn word"
Increase LRU cache size to prevent thrashing, invalidate on "learn word"

Fixes #136

I'm not sure what the motivation for the 4s invalidation time in the LRU cache was. My best guess is that it was necessary to make the "learn spelling" feature work, so instead of re-querying every 4 seconds I made `addToDictionary` reset the cache. I also made sure the cache is reset when you switch languages on macOS (this may have actually been a bug?)
paulcbetts/electron-spellchecker,paulcbetts/electron-spellchecker