Use iconv-lite to decode dictionary files

Closes #1.
Limit CSS properties of spelling highlight

Before, we had set the whole background, which interfered with things
search highlighting.  Now we only set the necessary properties to get
the red squiggly line.
Use dictionary to determine alphabet

The .aff file has a TRY line, but this doesn't necessarily list all the
letters.  Instead, we use some code that had been commented out of
Typo.js to generate an alphabet from the letters that show up in the
dictionary words.

Closes #2.
Version 0.0.2