Emoji in collections

Resolves https://github.com/jekyll/jemoji/issues/13.

/cc @parkr 
`generate` actually builds the site and process the Document. Without that line, I get the following error:

```
NoMethodError: undefined method `emoji_image_filter' for nil:NilClass
    /Users/garentorikian/Development/jemoji/lib/jemoji.rb:28:in `emojify'
    /Users/garentorikian/Development/jemoji/test/test_jemoji.rb:30:in `block in <class:TestJemoji>'
    /opt/rubies/2.1.4-github1/lib/ruby/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:413:in `instance_exec'
    /opt/rubies/2.1.4-github1/lib/ruby/gems/2.1.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb:413:in `block in create_test_from_should_hash'
```

`emojify` actually makes the emojification. Without it I get the following error:

```
TestJemoji#test_: Jemoji should replace collection content on generate.  [/Users/garentorikian/Development/jemoji/test/test_jemoji.rb:31]:
--- expected
+++ actual
@@ -1 +1 @@
-"<img class='emoji' title=':+1:' alt=':+1:' src='https://assets.github.com/images/icons/emoji/unicode/1f44d.png' height='20' width='20' align='absmiddle' />"
+":+1:"
```