Support more geometry symbols

[onomou]

It would be nice to have access to the geometry symbols like parallel/not parallel (∥ ∥/∦ ∦), parallelogram (▱ ▱), and (this might be wishful thinking) the double arrow over letters (used to signify line AB as opposed to the single arrow over ray AB and bar over segment AB).

[drlippman]

While a little clunky, you can get the line accent using overset(harr)(AB)