HipChat alert doesn't create emoticons

If I create a message in the alert like this:
`.message('({{ index .Tags "host" }}) {{ .Level}} - cpu idle: {{ index .Fields "usage_idle" | printf "%.2f" }}%')` doesn't work.``

Which gives, for example, a message like this:
`(avocado) OK - cpu idle: 85.41%`

The emoticons don't work. I think this is because, as per the [hipchat docs](https://www.hipchat.com/docs/apiv2/method/send_room_notification), the api assumes it is an `html` message. If we set the `message_format` to `text`, I think it will work.

Or it would be nice if hipchat detected that there is no html tags, and assumed text instead. But I won't hold my breath on Atlassian changing that, at least not quickly.

(If I have time to set up a go dev environment I'll submit a PR, but that might be a while since I'm not in that world right now.)

Locate the HipChat alert notification implementation and its tests, then read how the message payload is constructed. Reproduce the example with the `(avocado)` emoticon and verify that the resulting notification preserves it as text; done means the alert displays the emoticon correctly without breaking HTML-formatted messages.

Written by the indexing model from the issue text.