add Lookup and AllowedMethods methods, change custom NotFound semantics

[mna]

Hello,

I've used something like this fork of pat for a while, it adds two methods that can be very useful when building middleware chains:

• AllowedMethods : returns the allowed methods for a given path, useful esp. for a CORS middleware.
• Lookup : returns the matching registered handler for a given method and path, useful if in a middleware you want to check if the request actually has a handler before starting setting up the usual chain (e.g. logging, panic recover, cors, etc.)

I also made another change that may be a bit trickier to merge, I found it quite weird that setting a custom NotFound handler on the mux changed the way the 404 was returned. If there's no NotFound and a path has other methods registered, it returns a 405, and the 404 is only returned if there's no match and no other methods for the path. Once you set a custom NotFound handler, the behaviour changes and the 405 is not returned anymore (presumably it's because the custom not found should handle it? Still seems weird/unexpected).

So this PR changes the behaviour so that setting a custom NotFound still behaves the same way as without it - it's just called if no 405 was returned. A case could probably be made to add support for a custom MethodNotAllowed handler, but I didn't add this.

I'm fine with keeping this stuff in my fork if you're not interested in adding more API surface to the package, no worries.

Thanks,
Martin