Add "list_of(type)" and "dict_of(key_type, val_type)" validators

[sscherfke]

x = attr.ib(validator=instance_of(list)) is IMO not a very useful check, but x = attr.ib(validator=list_of(type)) would be.

In a similar fashion we could also add a dict_of(key_type, val_type) validator.

I can create a pull request if you like this idea.

[hynek]

We’re getting this request regularly. You may want to look into the discussion around #158 to what I’m envisioning if you’d like to supply a solution.