Feature Request: Proxy Support

[ryanewtaylor]

My help documentation, served at http://localhost:3000/ needs to make fetches to external resources via relative paths, in different environments (i.e., dev, integration, staging, and production). It would be very helpful to support proxying during local dev with sphinx-autobuild so that I can redirect requests the relative requests to the appropriate endpoint. I am thinking something similar to what webpack dev server does.

--proxy="context:'/rel/url1' target='/target/url1' secure=false"
--proxy="context:'/rel/url2' target='/target/url2' secure=true"
--proxy="context:'/rel/url3' target='/target/url3' secure=false"