Customize route failure response

[aturon]

Right now, Tide always returns an empty 404 response when there is no route match. This should be customizable.

Possibly blocked by #5, if we want to use the same configuration mechanism for this.

[aturon]

Also note that currently no middleware is applied to this fallback 404 responder. Worth thinking if that's what we want.

[tzilist]

@aturon Currently working on this. Thought process is as follows:

We can allow user's to specify a default handler. This function will run if no route match is found. The other thing i'd like to implement is an error handler akin to actix-web's, see here. I think between the two of them, that should give a pretty good start for handling any kind of failures that can happen during a request :) Let me know what you think!