Add support for defining attributes from environment variables

In container world we don't always have easy access to installer script and it's parameters but we do have easy access to defining new environment variables.

So I would humbly request a feature that would allow me to define something like
COMPOSER_CHANNEL=1
which would be same as giving --1 for the installer script. And to keep logic clean giving --1 should override environment variables.

Having all variables available as environment variables would ease use of composer installer in docker/Openshift containers.

br,
Tero Jankkila