[emoji] Added basic emoji feature
[emoji] Finished initial (rough) implementation
[emoji] Added a default CSS file to the presentation

This contains basic instructions for the emoji images.
[emoji] Supported ability to configure an emoji asset_root
[emoji] Created custom emoji-filter

It's presently the equivalent of the EmojiFilter.
[emoji] Updated emoji-filter to remove hard-coded width, height

This might look great on web pages, but it doesn't look that good in
reveal.js slides.

Plus-- users can provide their own CSS-- and removing these inline
attributes gives them better control.
[emoji] Updated emoji-filter to please rubocop.
Added support for emoji (:thumbsup:)

Merge branch 'emoji-again' into upcoming

* emoji-again:
  [emoji] Updated emoji-filter to please rubocop.
  [emoji] Updated emoji-filter to remove hard-coded width, height
  [emoji] Created custom emoji-filter
  [emoji] Supported ability to configure an emoji asset_root
  [emoji] Added a default CSS file to the presentation
  [emoji] Finished initial (rough) implementation
  [emoji] Added basic emoji feature