This uses the $_ENV superglobal, which is an empty array unless the
variables_order ini directive contains "E"; see
http://php.net/manual/en/ini.core.php#ini.variables-order.
Environment variables have lower precedence than input data passed via
other means, e.g.
$ echo '{{USER}}' | twigc --env
peets
$ echo '{{USER}}' | twigc --env -p USER=foo
foo