Ability to add aria-label property to linkit link

[jamesmacwhite]

This is similar to previous requests around additional HTML attributes, but one that could be use is aria-label on links.

At the moment the only way seems to be creating your own custom link type, but I don't really want to have to create a whole different type for adding a single field. Basically similar to the custom text option but being able set an aria-label value.

Cases for it might be if you are using short action labels on buttons such as "Book online" but want to have a better label for screen readers e.g. book online through xxxx