Added GIF search plugin

This is basically similar to the Google Image Search command, but uses the [Giphy API](https://github.com/giphy/GiphyAPI) to return a random GIF in the top ten results for a given query.

The API is currently in beta, and has a single public API key `dc6zaTOxFJmzC` that will need to be added to the `api_keys` config setting:

```json
{"api_keys": {"giphy": "dc6zaTOxFJmzC"}}
```
If this ends up being heavily used, it is recommended to contact api@giphy.com for a unique API key.