Enhancement: Custom template for message body

[Rhobal]

I implemented an uib-progressbar inside a toast to show the progress of a background processing operation. Due to issue #190, I had to modify the standard template and use a more flexible template showing an uib-progressbar in certain conditions.

My feature request is to support an additional option such as messageTemplate in the customizing options of the toast and then ng-include this template instead of the message div if the option is set.