Streaming mode for downloading all rows as a CSV (#315)

* table.csv?_stream=1 to download all rows - refs #266

This option causes Datasette to serve ALL rows in the table, by internally
following the _next= pagination links and serving everything out as a stream.

Also added new config option, allow_csv_stream, which can be used to disable
this feature.

* New config option max_csv_mb limiting size of CSV export

tests/fixtures.py

@@ -71,6 +71,13 @@ def app_client_larger_cache_size():
     })
 
 
+@pytest.fixture(scope='session')
+def app_client_csv_max_mb_one():
+    yield from app_client(config={
+        'max_csv_mb': 1,
+    })
+
+
 def generate_compound_rows(num):
     for a, b, c in itertools.islice(
         itertools.product(string.ascii_lowercase, repeat=3), num