More granular control over presenters

Changed

• Presenters should check if a presentation model has data for themselves. This is no longer handled by the
  PresenterMiddleware. This allows more granular control over presenters. For convenience, there's a HasData trait
  available.
• Trying to set a variable with a non-string key will cause type errors instead of silently ignoring the variable.

First release

This is the first release of Shoot! Feedback, issues, and PR's are more than welcome.