Suggestion: Add support (syntax) for LaTeX alphabetic indices

As we have been using more and more of MMD for creating a LaTeX version of our handbook, we came across the need to add an index (an alphabetic list containing the main terms) to the document. While MMD supports syntax for glossaries and acronyms, an equivalent for indices is currently missing.

So I would suggest the addition of a special syntax for indices, which might look like `[>>syntax]` or `[!syntax]` or anything like that, and is then converted to `\index{syntax}`

As a sidenote: we embed MMD into our writing app, so we could define and handle our own syntax, but we would prefer some kind of *standard*, of course.

Thanks,
Alfons