Allow to define bbcode properties in markdown

[daenvil]

Right now, some syntax elements are missing a way of setting some of their properties. For example, images have width, height, etc. BBCode has to be used to set those properties.

The attribute syntax described at CommonMark could be implemented to fix this.

For example:

![img](res://some-image.png){width=400 height=500}

The same could be used to set custom heading anchors, e.g.:

# Some heading {#some-custom-anchor}

This is not a priority since most sites that use markdown don't allow this syntax. It would just be a nice to have so we don't have to use BBCode inside Markdown for things like this.