Fixed #1786: Add configurable type hints.

This adds the option of giving, in addition to the type of the default
value, hints about permissible types for configuration values.

tests/roots/test-config/conf.py

@@ -0,0 +1,32 @@
+value1 = 123  # wrong type
+value2 = 123  # lambda with wrong type
+value3 = []  # lambda with correct type
+value4 = True  # child type
+value5 = 3  # parent type
+value6 = ()  # other sequence type, also raises
+value7 = ['foo']  # explicitly permitted
+
+class A(object):
+    pass
+class B(A):
+    pass
+class C(A):
+    pass
+
+value8 = C()  # sibling type
+
+# both have no default or permissible types
+value9 = 'foo'
+value10 = 123
+
+def setup(app):
+    app.add_config_value('value1', 'string', False)
+    app.add_config_value('value2', lambda conf: [], False)
+    app.add_config_value('value3', [], False)
+    app.add_config_value('value4', 100, False)
+    app.add_config_value('value5', False, False)
+    app.add_config_value('value6', [], False)
+    app.add_config_value('value7', 'string', False, [list])
+    app.add_config_value('value8', B(), False)
+    app.add_config_value('value9', None, False)
+    app.add_config_value('value10', None, False)