Support for something like ruby tags?

[thomasp85]

Could be a variation of the link/image tag system, using arrows to show where to place them

↑[吾輩](わがはい)

would turn into the equivalent of

吾輩 （ わがはい ）

[thomasp85]

Maybe this is too concerned with styling (the arrow part). In the spirit of markdown it might be better to not use an arrow but something that doesn't prescribe where the annotation is placed