Custom Filters Idea

[satoved]

Custom filter problem
When I override filters view and try to create some custom filters (e.g. select2 with ajax sources options), they get cleaned by cleanFilters() function.

I suggest adding another Filter::custom() type, that won't be filtered.

Or maybe Filter::multiselect()->noClean() or something like that.

Breaking change idea
Ideally for v2 I would like to extract Filters into their own classes, cause this "isSelect()", "isMultiselect()", etc. all over the code is not good.

That will allow user to create his own implementation of filters, e.g. Select2Filter, that will clean itself, render itself, etc.

Willing to help with pull requests, but let's discuss first?