Autocompletion of reference link ids

[dsanson]

Eventually, I'd like to add context sensitive autocompletion of reference ids.

So, for example, if there is something like

Blah blah blah [blah][my favorite link]. Blah blah blah [blah][my least favorite link]. And [an implicit link][]. And [an even more implicit link]. And a footnote.[^footnote]

And I then start a line with something like

[my

and hit CTRL-X CTRL-O, it should suggest both [my favorite link]: and [my least favorite link]:. Likewise for footnotes, if I start a line with something like

[^foot

it should complete to [^footnote]:.

Of these, I think footnote id completion should be easiest to implement, since the format for the references, [^id] is easier to scan for than the ids for images and links.