Python: need a way to execute a count with an extra where clause

[simonw]

I need this for #304. I'll probably add this to the .execute_count() method as where= and where_args=.

[simonw]

I'm going to rename .execute_count() to .count_where() - but I'll keep .execute_count() in the codebase since it's a documented API at https://sqlite-utils.datasette.io/en/3.13/python-api.html#count and I don't want to do a major version bump.