Header anchors

[luckydonald]

Basically the closed but unsolved #122.

# Some Title

should generate

<h1><a name="some-title" aria-hidden="true"></a>Some Title</h1>

So linking to #some-title would work.

[jonschlinkert]

@luckydonald it should be pretty easy to create a plugin for this. I think there might be one already. Leaving this open as a reminder to myself (or someone else) to add a link to one here, and/or a link to how to create plugins for remarkable.