Render a custom template as flash, with an associated controller

[sebn]

This PR allows you to define flash messages in templates, and to render them with whatever controller you want.

This has some advantages:

• You can put custom markup in your templates: images, titles, links...
• You can bind some template tags to custom controller actions (e.g. "undo")
• If you think flash is a "view" thing (as I do), you can move it from the control layer to the view one
• You can still put a simple text message with some helper handling all the default markup

Of course, the text-oriented API still works.

I have a working implementation, but it may deserve some refactoring:

• Extract a message model, maybe with some isTextMessage() method?
• Maybe find a better name to the flash() method... flashTemplate()? Any idea?
• DRY the route mixin
• Don't have to create a custom object when calling from a controller... by providing some mixin too?

WDYT?