Add rework.func to easy define functions

Rework filters sometimes is to compilated for newbies, we have `rework.mixin` to easy define own properties (it allow us to have clean code in Rework and allow end-users to easy define they own project-specific properties).

When I wrote a `rework.inline` I think, that it has a lot of common code with `rework.url`. So DRY god ordered me to move common code to `rework.func`.