Provide access to AWS Stage Variables

KMS is a good solution for storing secrets, but it's not free. During development I would rather use stage variables instead, but apparently there is no way to access them right now.
There are at least two possible solutions that come to my mind:
1. Pass in HTTP header fields. Either as a single field with JSON or as a bunch of fields like `X-AWS-STAGE-VARIABLE-1: Name=Value` whichever seems more idiomatic (I'm not an expert in this area).
2. Transfer to environment variables before starting lambda server process - this one will require process restart if stage variables changed and lambda is recycled.

This is sort of a side topic for https://github.com/apex/up/issues/90 and seems to be related to https://github.com/apex/up/issues/200.