Querystring support (WIP)

[j4mie]

Fixes #13

Having done a bit of work on this, I'm not totally sure this is a good idea.

1. Where should the querystring values be available? Just in templates? Or should the JSON handler have access to them too?
2. If the JSON handle should have access, how will that work? Currently the SQL is just bound with the params from the URL - will the URL and query params need to be namespaced or should they just be smashed into the same dict? Which should take precedence in that case?
3. At the moment, the query is a dict where the values are lists, because ?foo=bar&foo=another is valid. Do I need to implement some sort of MultiValueDict to make this nicer?

This all seems like it might be a bit overcomplicated.. 🤔