Add async argument to wp_register_script and wp_enqueue_script

[nickciske]

Though defer is a mess and async isn't supported in older IE browsers, it seems like allowing async with a default of false gives plugin and theme authors the choice to use it if it's right for them... without hacky workarounds.

Some background:
​http://www.html5rocks.com/en/tutorials/speed/script-loading/

[SergeyBiryukov]

Duplicate of #12009.