Add binding for App.filter

[driesdewinter]

Hi,

In my javascript application I need to keep track of the number of open connections. I found that the C++ library already has the filter function for that, so I added a wrapper for it.

I tried to follow the examples of the other wrappers as much as possible, and also added a brief documentation. Could you review and consider integrating this change in the master branch?

Thanks in advance,

Dries De Winter.

[uNetworkingAB]

looks good