Replaces emoji codes (like :smiley:) for emoji images (like 😃) or unicode characters (like 😃).

$ npm install emojis
(...)
$ node
> var emojis = require('emojis')
undefined
> emojis.replaceWithUnicode('I :heart: you!')
'I ❤️ you!'
> emojis.replaceWithHtml('I :heart: you!', 'http://example.org/images/')
'I <img class="emoji" width="20" height="20" src="http://example.org/images/heart.png" alt="heart"> you!'

The emoji codes used are the ones on Emoji cheat sheet.

Their images are also provided here for you to use (images.zip), but consider its copyright.

One way to align the emoji image within the text would be using align="absmiddle", just as GitHub does, but I didn't do it because it's obsolete.

A suggestion is to use the following CSS:

.emoji {
  margin-bottom: .25em;
  vertical-align: middle;
}