Add --build-env option.

Adds a new option which can be used to set an array of build
environment variables. Internally these are injected into
the docker run command for the build environment and can be
used for setting up a local proxy for example:

 delorean --build-env http_proxy=http://192.168.1.1:3128...
Merge pull request #3 from dprince/build_env

Add --build-env option.