Command line option for custom headers

[ivan-gomes]

Resolves #205.

configurable-http-proxy supports a wide range of configuration options, but it is not practical for it to cover all use cases. An example of an uncovered use case is the Strict-Transport-Security response header. Instead of adding yet another configuration option specifically for this header I propose we implement a --custom-headers <headers> configuration option that allows for user-specified headers to be added to all proxied requests. Upstream projects like https://github.com/jupyterhub/zero-to-jupyterhub-k8s can expose the option to be passed to configurable-http-proxy, for example through the Helm chart.

[rgbkrk]

My only main question is whether these headers are applied after other headers being set and if we should document so here.

[ivan-gomes]

Custom headers are indeed applied after all other headers using the proxyRes callback.

[minrk]

Makes sense! Minor comments on the implementation.

[rgbkrk]

Min's points are solid so I've set myself to pending review (though GitHub still shows my approval).

[ivan-gomes]

Implemented @minrk's points. Thanks for the guidance. @rgbkrk would you mind re-approving?

[minrk]

Thanks! Sorry for being so slow