Add a SelectFunc(filter) selector that just delegates to the given filter function

[jwodder]

This would let users create custom selectors (so that, say, they can combine their logic with other selectors) without having to create a custom Selector subclass.

[isidroas]

A little overlapping with #8 . IMHO I prefer sub-classing because is more specialized and easier to document.

[isidroas]

I withdraw what I've said. The function signature is so simple that it is Ok to pass it as an argument. That is more Pythonic. Similar to the first argument of filter().