Add @context support for JStachio module

[agentgt]

@jknack

JStachio added support for loose key->value stuff like request attributes. Originally this was a pain in the butt if you had say CSRF token you would have to add it manually to all models.

More info on that can be found here:

https://jstach.io/jstachio/io.jstach.jstachio/io/jstach/jstachio/context/package-summary.html

So now say something like CSRF token is bound to Context.getAttributes you can access it in JStachio like:

{{@context.csrf}}

This PR makes that possible.

[jknack]

Can you add a minimal example to the jstachio jooby doc?

[agentgt]

@jknack Will do. I ran out of time right before the holidays and meant to leave it in draft mode for a little longer.

[jknack]

@agentgt Ok, going to merge and release. We can update doc later.