Allow user to pass config file via CLI

[jsignell]

It should be possible to pass a yaml file to anaconda-project using --file this would be a nice first step for getting off anaconda-project.yml as a mandatory file, allowing the user to put these extra fields in an environment.yml instead for instance.

[jbednar]

Agreed. Seems like a simple and appropriate extension.

[AlbertDeFusco]

There is a PR open to support environment.yml files directly #275

Could you explain a little more about how you might use a --file argument? Would the current working directory be the place where the envs are prepared?