Suggestion: Add config-by-environment-variable support

[sbrl]

I want to use a forward proxy for the various package managers on my network to reduce bandwidth usage - particularly because:

• I have a continuous integration server, which I want to look at Dockerising builds at some point
• I build a number of Docker containers regularly

I run a cluster of Pis running Hashicorp Nomad + Consul, so I intend to Dockerise angry-caching-proxy. This isn't necessarily a problem, but it would be very nice to have the ability configure angry-caching-proxy via environment variables. This would allow me to integrate the configuration into the Dockerfile I'm writing.

This is all a long-winded way of suggesting that environment variables such as ACP_DIRECTORY, ACP_PORT, etc be used to configure angry-caching-proxy.