Optional param dict for channel subclasses

Hi, I have a usecase where it is very helpful to be able to pass initialization parameters to custom lookup channels. I implemented support for an optional 3rd parameter for custom channel declaration in AJAX_LOOKUP_CHANNELS. 

This parameter is a dictionary that gets passed to a custom lookup channel's __init__() method. Having this ability makes custom lookup channels more reusable and more DRY.

If this sounds useful to you, feel free to merge it in. I have also updated the README to reflect this convenient option.