Add spawn_restricted_if

[funnyboy-roks]

Add a spawn_restricted_if method to the CommandExt that accepts an Option<Rules> and only spawns restricted if those rules are Some.

Command::new("foo")
    .arg("bar")
    .spawn_restricted_if(optional_rules)

spawn_restricted_if is maybe not the greatest name. We may want to come up with something better.

This would allow us to get rid of some ghastly code:

let mut cmd = Command::new("foo");
cmd.current_dir(cwd);
let output = if let Some(compile_rules) = compile_rules {
    cmd.spawn_restricted(compile_rules)
} else {
    cmd.spawn()
};