Support fallback or loading_view prop in ProtectedRoute

[tqwewe]

Is your feature request related to a problem? Please describe.
The new ProtectedRoute using transition behind the scenes is great. However there doesn't seem to be a way for users to customize the fallback view for when the condition is still being resolved asynchronously.

Describe the solution you'd like
The ProtectedRoute component should have an optional prop for allowing a view to be rendered while the condition is loading. This allows for nice loading screens etc.

[gbj]

Should be a 1-2 line change to add a fallback and pass it through to the Transition -- Are you interested in making a PR?

[tqwewe]

Yep sure! I'm a little confused about what to name the new prop. fallback in the Routes component means the view to render if the route isn't matched. Are we fine to use then name fallback in the ProtectedRoute for the transition fallback?