Added column__not=blah filter

Closes #148

datasette/utils.py

@@ -332,6 +332,7 @@ class Filter:
 class Filters:
     _filters = [
         Filter('exact', '=', '"{c}" = :{p}', lambda c, v: '{c} = {v}' if v.isdigit() else '{c} = "{v}"'),
+        Filter('not', '!=', '"{c}" != :{p}', lambda c, v: '{c} != {v}' if v.isdigit() else '{c} != "{v}"'),
         Filter('contains', 'contains', '"{c}" like :{p}', '{c} contains "{v}"', format='%{}%'),
         Filter('endswith', 'ends with', '"{c}" like :{p}', '{c} ends with "{v}"', format='%{}'),
         Filter('startswith', 'starts with', '"{c}" like :{p}', '{c} starts with "{v}"', format='{}%'),

tests/test_app.py

@@ -254,6 +254,10 @@ def test_paginate_tables_and_views(app_client, path, expected_rows, expected_pag
     ('/test_tables/simple_primary_key.json?content__exact=', [
         ['3', ''],
     ]),
+    ('/test_tables/simple_primary_key.json?content__not=world', [
+        ['1', 'hello'],
+        ['3', ''],
+    ]),
 ])
 def test_table_filter_queries(app_client, path, expected_rows):
     response = app_client.get(path, gather_request=False)