Enhance 'envs' and 'tags' DSL

This makes it possible for users to write

    env :development
    env :staging, :production
    envs :development, :test

with the result being `[:development, :staging, :production, :development, :test]`, and

    tag :foo
    tag :bar, :baz
    tags :foo, :qux

with the result being `[:foo, :bar, :baz, :foo, :qux]`.