MD051: Allow link fragments to kramdown block IAL and span IAL ids

[randallpittman]

Related: #570

kramdown lets you give ids to blocks and spans with the inline attribute list syntax. It would be great to not have false positives for MD051 when linking to an id of this syntax.

Example syntax

This is the paragraph I want to link to.
{:#the-paragraph-id}

Later on, I'd like to link the [the paragraph I wrote earlier](#the-paragraph-id).

[DavidAnson]

Thanks for the related link. This doesn't look too bad to allow. It's funny these two systems don't even agree on the same syntax for this - tho it's pretty clear they knew about each other.