Make css and className optional for element and elementKeyed

[jvliwanag]

Fixes#18

I can also update the example if need be to remove className and demonstrate it can be used without.

[i-am-the-slime]

@mjrussell Any chance of this making it in?

[mjrussell]

Yeah it looks like a good improvement. @jvliwanag do you want to fix the conflicts?

[mjrussell]

Updating the example for with and without would be super helpful but could be done later

[i-am-the-slime]

@mjrussell I don't think @jvliwanag really does PS anymore, so we might have to take this over.