Add a catch-all handler 2nd version

[Tibbel]

Here is the modified catchall-function
example added
the code moved to routing.h
only the setter remained in app.h
the integer return is removed
i personally don't need the default parameters:
without crow::response the catchall can take no effect
and without crow::request you know nothing about the request
i only imagine one case, this is to set custom error pages.
so i don't thing default parameters have any big benefit here.
"#ifdef CROW_CATCHALL" was only for me, to find my own modifications feel free to replace/rename it

[Tibbel]

It would be a lot nicer if CROW_CATCHALL was like CROW_ROUTE as in you just use the macro to create a catchall route.

for me macros are more c'ish and not really c++ i use it only for workarounds, but here is one:
#define CROW_CATCHALL(a, f) a.catchall_handler( f );

[The-EDev]

HI, Thanks for taking the time to change the original PR.

I've noticed there are large differences between this PR and how Crow is written, so instead of merging into master I'll merge it into a feature branch and work from there to make sure everything is uniform.

I'll also open an issue to keep track of any discussion.

Thanks again for your work and for bringing this feature up.