Expose level_tag property on messages.

Exposing the level name (e.g. "info") makes it possible to prepend
something to the class name. For example, Twitter Bootstrap has
an alert-info class. This class can now be added to the message
using `class="alert-{{ message.level_tag }}"`.
Because the level_tag was on the end of the `tags` property, it
could not be used in this fashion when extra_tags were given.