Plack::Middleware::Static: add ability to pass $env to callbacks

Change the callback interface for static middleware so that if you
specify a true "pass_env_not_path_info" argument you'll get $env as an
argument, not $env->{PATH_INFO}.

Some middleware might want to look at other things in $env, or consult
some object we previously stuck in $env instea dof just looking at
$env->{PATH_INFO}. This adds the ability to easily do that.