Improvements (#33)

1. Skip recording undefined variables
2. Parse `in` correctly if it is not imported.
3. base64.decode
4. Handle `with` modifier for qualified data and input.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/parser/cases/some/some.in.yaml

@@ -134,6 +134,7 @@ cases:
   - note: same-line
     rego: |
       package test
+      import future.keywords.in
       x = y{
           some a b in {4, 5}
           [1, 2, 3][a] == 3