Allow setting different namespace root

[shioyama]

Currently the root for any loaded namespace is the loader instance. But someone may want to set autoloads on another (named or anonymous) module, and there is no reason Im should not support this.

With this change, it is now possible to pass a root when initializing a loader. This root (a module) is then used in place of the loader itself as the root of the (auto)loaded namespace.

e.g.

mod = Module.new
loader = Im::Loader.new(root: mod)
loader.push_dir(...)
loader.setup

Now, autoloads are set on mod instead of loader. Passing Object as root here makes Im act like Zeitwerk.

[shioyama]

I need to update the readme and add changelog entry etc, but otherwise this should be a simple change.