Add URLConfig preprocessor to warn on common mistakes

[jmosbacher]

We should add a list of common mistakes made when preparing a URL config and add a URLConfig preprocessor that will warn the user if any of these common mistakes are identified in the URL.

Some examples:

• Not adding &as_list=True when a list should be returned from xedocs
• Not using the resource:// protocol to fetch files on corrections that return filenames
• Not sorting the values when building an array from the result of xedocs protocol.
• referencing a correction that doesnt exist in the xedocs protocol

We should probably organize these issue by protocol, so eg have a list of functions for each protocol. Each function should be called with the argument and kwargs of the URL and optionally issue a warning to the user if a possible mistake was identified. Alternatively we could just register all the functions as preprocessors