--limit= CLI option for setting limits

[simonw]

#264 calls for four new datasette limit options, on top of the two existing ones:

• --max_returned_rows
• --sql_time_limit_ms

These are already clogging up datasette serve --help a bit.

How about this syntax instead?

datasette --limit max_returned_rows:100 \
    --limit facet_timeout_ms:500 demo.db

Then we can add as many new user over-rideable limits as we like without clogging up --help too much - though it would be good to have a way of optionally listings their documentation as well.

[simonw]

I'm going to add a /-/limits page that shows the current limits.