Spatula

- add support for new versions of lxml and Python

* add `Page.accept_response` method that can be overriden to trigger custom retry logic
* add preliminary spatula.config for setting/overriding global defaults
(this feature is not yet considered stable, it likely will be modified before 1.0)

* update click dependency

* fix for `--rmdir` not recreating directory

* add `--rmdir` flag to `spatula scrape`