bring back require.extensions

One of the complaints I heard about js-yaml is that you can't `require('./something.yaml')` like you can with json. I mean, you used to have that feature, but it was removed in 9c10316ddf68c686f396a9c8680db048d423245d.

The issue with it is not usage of require.extensions itself, but rather auto-registering by default, which is [unexpected](https://github.com/jashkenas/coffee-script/issues/2323).

I suggest to bring back that behaviour, but make registering explicit, with a syntax like `require('js-yaml/register')`. It is becoming kinda standard, at least [coffee](https://github.com/jashkenas/coffee-script/blob/master/register.js) (with [redux](https://github.com/michaelficarra/CoffeeScriptRedux/blob/master/register.js)) and [streamline](https://github.com/Sage/streamlinejs/blob/master/register.js) is doing that. See also discussion in https://github.com/jashkenas/coffee-script/issues/3141#issuecomment-26774612 about this behaviour.