filters_from_request plugin hook, now used in TableView

- New `filters_from_request` plugin hook, closes #473
- Used it to extract the logic from TableView that handles `_search` and
`_through` and `_where` - refs #1518

Also needed for this plugin work: https://github.com/simonw/datasette-leaflet-freedraw/issues/7

datasette/hookspecs.py

@@ -89,6 +89,17 @@ def actor_from_request(datasette, request):
     """Return an actor dictionary based on the incoming request"""
 
 
+@hookspec
+def filters_from_request(request, database, table, datasette):
+    """
+    Return datasette.filters.FilterArguments(
+        where_clauses=[str, str, str],
+        params={},
+        human_descriptions=[str, str, str],
+        extra_context={}
+    ) based on the request"""
+
+
 @hookspec
 def permission_allowed(datasette, actor, action, resource):
     """Check if actor is allowed to perform this action - return True, False or None"""