Add BUILDER_CONFIG environment variable

[terhorstd]

To make the configuration file select-able from the environment, this PR adds the BUILDER_CONFIG variable. For existing setups nothing should change, as the default value remains the same. It however enables users to keep different Builder configurations around (or in a repository) and point Builder to the one to use.

fixes #65

[finnerty-fzj]

.buildrc is also mentioned in Install.md and README.md without editing. Would suggest at least mention of possibility of using an alternative file in mentioned in the README.md.

Merge request also has minor unrelated changes, is this ok with project policy?

[finnerty-fzj]

Nothing more to say.

[terhorstd]

Thanks for the quick review!