Allow /validate to returns headers with claims

[artagel]

If #95 is implemented, we can provide a fully customizable interface for sending headers down to the proxies application.

[bnfinet]

This is certainly something we've talked quite a bit and that I'm fully in support of.

Were you able to look at the performance for mappedClaims and returning headers from /validate in #90?

The SLA we've set is "less than 1ms" but my hope is to stay closer to the 0.5ms we're at now (on reasonable hardware).

Really /validate performance should be a unit test with mocking of the IdP but we're not there yet.

[artagel]

No, I'll try to find a good way to measure the performance impact.

[artagel]

After discussion on IRC, we want to accomplish the following:

1. Add custom claim support
2. Add it to the JWT instead of the user struct
3. pass them down to nginx using X-Vouch-IdP-Claims-"claim"
4. Add config item vouch.headers.claims

[bnfinet]

fixed in #102