Add support for nested parens, e.g. "Definition (O(n) runtime)."

[isovector]

Took this todo out of the source. pandoc-theorem looks like an excellent project, and this is a reminder to myself for something to work on during #hacktoberfest

[sliminality]

Thanks for the kind words, and for opening this issue! Hacktoberfest is over, but I'd love to get around to this myself (in my copious spare time 😅) and contributions are always welcome. :D