RTL-friendly align property for drawer

[keanulee]

Fixes #37

cc @blasten

[blasten]

There appears to have a timing issue when _computePosition is called. I tried setting dir=rtl in the doc element and the position remained left. We might be able to set the position by calling _setPosition from the debouncer in resetLayout wdyt?

[keanulee]

Ah yes, I can get drawer's computed direction (e.g. 'rtl') only after it is attached, which computed properties does not support.

[frankiefu]

lgtm