add update plugin

Calls git pull and bundle update before sending HUP to the bot
process. Requires the user that sends '!update' to be op of the channel.

```
02:14 <@relekang> !update
02:14 < itera-bot> Current branch master is up to date.
02:14 < itera-bot> From github.com:relekang/itera-bot
02:14 < itera-bot>  * branch            master     -> FETCH_HEAD
02:15 < itera-bot> Your bundle is updated!
```