Make it possible to customize various facet settings

[simonw]

The new Facets implementation from #255 includes several hard-coded settings which should be made configurable somehow:

Number of rows to return in a facet (maybe this should also be an option that can be set via quersytring argument, e.g. ?_facet=qSpecies:40):

datasette/datasette/views/table.py

Line 539 in 9959a9e

FACET_SIZE = 20

Time limit for executing a facet:

datasette/datasette/views/table.py

Lines 559 to 562 in 9959a9e

	facet_rows = await self.execute(
	name, facet_sql, params,
	truncate=False, custom_time_limit=200
	)

Maximum unique values returned in order for a column to be suggested as a facet:

datasette/datasette/views/table.py

Lines 646 to 647 in 9959a9e

	# Detect suggested facets
	FACET_LIMIT = 30

Time limit for calculating if a column should be a suggested facet:

datasette/datasette/views/table.py

Lines 664 to 667 in 9959a9e

	distinct_values = await self.execute(
	name, suggested_facet_sql, params,
	truncate=False, custom_time_limit=50
	)

[simonw]

Docs: http://datasette.readthedocs.io/en/latest/limits.html#default-facet-size