Public Context

[willcosgrove]

I wanted to open this draft PR so we had a space to talk about the API here.

• Internally I've called this data user_context
• The public API to access it is just at context
• To allow for setting user_context through the call method, I've needed to rename the existing context keyword argument. So I made both it and parent use our internal naming scheme _context:, _parent:
• context is by default an empty Hash

What are everyone's thoughts?