Added specs for EmojiFilter.

Note, however, that one spec is commented out; lines 209-211 should
verify that enclosing an emoji within a `<tt></tt>` tag pair should
*not* expand the emoji, following the example of [this
spec](https://github.com/jch/html-pipeline/blob/master/test/html/pipelin
e/emoji_filter_test.rb#L38-L43) in the current HTML::Pipeline `master`
branch. However, uncommenting that spec will cause it to fail. Further
investigation?

RSpec: 703 examples, 0 failures, 1 pending
RuboCop: 179 files inspected, no offenses detected