Add support for PHP mongodb extension

[hussainweb]

This adds support for the mongodb PHP extension which replaces the old mongo extension. The default is still the old extension which can be changed with a variable.

Again, since the default is the extension which is currently installed, I hope this counts as a minor feature and gets in quickly.

[fideloper]

Any reason not to assume people will use the more modern extension and go with that? That'd be my preference for Vaprobash, but I haven't touched Mongo in years.

[hussainweb]

The modern extension is not backward compatible, AFAIK. If you have an app built with the old extension, you likely want that. Changing the default would count as a breaking change. Also, in contributing doc, you say that this is using semver which would mean changing the default should go to 2.0.0.

I thought this would be a good fit even for a minor release and when it's time for 2.0.0, we just change the default or even remove support for old extension.