Allow adding custom headers when fetching a JWKS document.

[dmaze]

We have an environment where we need to provide custom headers to HTTP GET a JWKS document (more specifically, an l5d-dtab: routing header for the linkerd service mesh). This PR adds an optional headers: field to the JwksClient configuration, which allows us to do:

import jwksRsa from 'jwks-rsa';

jwksRsa.expressJwtSecret({
  jwksUri: 'http:https://service.name/jwks.json',
  headers: { 'l5d-dtab': '/foo=>/bar' }
});

The headers field is optional and the default is to add no additional headers; this should not introduce any compatibility issues for existing code.

[dmaze]

This PR is essentially identical to #77 (and its predecessor #64).