Allow users to pass DOM element instead of ID

Vega [allows you pass a DOM element or an ID](https://github.com/vega/vega/wiki/Runtime#basic-use), and so should Cedar.

May no longer need to [check if it's a valid id](https://github.com/Esri/cedar/blob/cfcd287ad24a956fb126b3feaa2c17886c1f6c2e/src/cedar.js#L347-L350), or at least only check if it's a string. Would impact [how height/width are calculated](https://github.com/Esri/cedar/blob/cfcd287ad24a956fb126b3feaa2c17886c1f6c2e/src/cedar.js#L586-L587). Hoping that removing or replacing those would lessen direct dependence on d3.