Use thing-at-point for citar-markdown-key-at-point

[localauthor]

This commit aims to fix a problem with the current version of citar-markdown-key-at-point, which is that it returns the closing bracket with the last citekey of a given markdown citation, such that [@Smith2012] returns Smith2012] in the Embark indicator.

Before resorting to thing-at-point, I tried editing the split-string regex in final line of the current code, to include a closing bracket, like so: (cadr (split-string str "[]@;]"))))). This nicely trimmed the closing bracket from the returned string.

But for reasons I don't understand, that change wreaked some strange havoc, leading Embark to recognize anything between brackets as a markdown citation. Not a great outcome.

I had some tinker experience with thing-at-point, so voila, an attempt at a bugfix. Interested to know what you think. Surely there's a simpler solution.

In any case, thanks for a great package. Using it daily and enjoying tracking its development, learning a lot from the conversations here between you and others.

[bdarcus]

Thanks @localauthor!

@roshanshariff - this is related to #395; shall I merge and you rebase?

One of the tricky issues with pandoc citation references is they aren't always in brackets.