Add HexHook to allow "!pkg" to search hex.pm

**I stole the idea from Zor in IRC**

When a command is issued in the format "!pkg SOME_TERM" the top 3
results will be displayed in the channel. One message per result.

To allow for the results to be displayed, a JSON decoder is required,
Poison has been added as a dependency to allow for this.

The description has been split into newlines and only the first is
displayed to avoid spamming the chat.
You need to add fallback case, otherwise we'll crash. And the query should still be passed through `String.strip` and encoded with `URI.encode_www_form`.