add support for custom HTTP handlers / interceptors

Hi

i have implemented the solution for custom HTTP handlers as described [here](https://github.com/atomify/atomify/issues/41). Could you please have a look at this and - if ok - integrate this into master? For the project i am currently working on this feature is really important as we have to serve some static content differently than atomify does by default (mainly for back end webservice proxying).

Thanks, Andi