Faster encode technique

Conflicts:
	lib/rumoji/emoji/symbols.rb

Ignore arbitrary strings wrapped in colons

fix colon with emoji problem

This is a fix for #7.
In addition to the updated iterator, I've adjusted the numeric emoji to
use accurate unicode codepoints.

Make decode regex work with codes that have dashes in them

Handle some edge cases with "possible emoji"

Don't strip characters that match the first codepoints in multi-codepoint emoji in Rumoji.encode_io

[ci skip]

Fixes #1

Support lookup for emoji with multiple codepoints.

Back Rumoji.encode with Rumoji.encode_io. Thanks to @jakeboxer.

A continuation of #3

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.