http_proxy is mapped from https_proxy

[scheetz]

https://github.com/cncd/pipeline/blob/master/pipeline/frontend/yaml/compiler/option.go#L234

Is there a good reason http_proxy is copied from https_proxy? It makes it problematic to correctly reference a local squid proxy used to rewrite request headers and expire certain urls to force refreshing.

[tboerger]

There is already a PR the would fix it at #48.