Support lookup for emoji with multiple codepoints

In addition, this code rewrites encode to just defer to encode_io. This
is an attempt to fix #2, though I believe the lookahead code can and
should be refined.
Don't lookahead for emoji, instead store previous potential emoji

A continuation of #3
Merge pull request #3 from mwunsch/multiple_codepoints

Support lookup for emoji with multiple codepoints.

Back Rumoji.encode with Rumoji.encode_io. Thanks to @jakeboxer.