Build: Upgrade to grunt-jquery-content 0.12.0

(cherry picked from commit 62861cec1cd731de9f24f15a0e214131a7fb1b89)
Build: Upgrade to grunt-wordpress 1.1.0 and grunt-jquery-content 0.12.1

(cherry picked from commit cfb281e6eac9eaea50f9076df1a42edf962de48b)
Tabs: Document extension

Fixes gh-336