Add message_template option to HipChat notifier

In this pull req I add `message_template` option for HipChat notifier.
It allows users to customize messages from the notifier.

---
`message_template` option must be Proc with exception as an argument.

Example:

```ruby
Whatever::Application.config.middleware.use ExceptionNotification::Rack,
  :hipchat => {
    :api_token => 'my_token',
    :room_name => 'my_room',
    :message_template => ->(exception) { "Exception: #{exception.class}: #{exception.message} on #{exception.backtrace[0..3].join("\n")}" },
  }
```