Add Javascript tests for non-UTF8, non-ASCII, and base64 extras

[pictographer]

9:58 AM] CurryHoward: I think everything is resolved, assuming it's accurate that 'base64' accepts both URL-safe and regular base64. We should change the example JS code in tagged-base64 to use the updated fromTaggedBase64 above (which removes the problematic ASCII conversion) and consider adding some tests:

• test for decoding non-UTF8 binary data
• test for decoding non-ASCII data
• test for decoding a TaggedBase64 string that contains - and _
  (CurryHoward is @jbearer)