Adds hook for devtools extension

[bryceosterhaus]

Let me know if you can foresee any issues with these changes.

Devtool extension is still a work in progress, but having this in Metal will help speed up development of the extension.

See https://github.com/bryceosterhaus/metal-devtools for reference

[eduardolundgren]

LGTM. It shouldn't cause any issue since it's just a optional hook. Let's see if @mairatma has any input on that, otherwise we can merge and release a patch with it.

[eduardolundgren]

I am going a head and merge it and if @mairatma has any input we can fix it.