Handles options.origin as function

[petuomin]

Makes it possible to pass a function as options.origin. The function gets request object as parameter and should return the string for 'Access-Control-Allow-Origin' header, or false to prevent the header from being set.

There is a simple example in the tests.

[rehno-lindeque]

:+1: Although the documentation claims...

Can also be set to a function, which takes the request as the first parameter.

...it looks like this change is required to actually implement it.

[evert0n]

Thanks and sorry it took long to look at it.

[rehno-lindeque]

Thanks for merging