Author List block: Sort vertically or horizontally

[Ken Gagne]

Is your feature request related to a problem? Please describe.
The Author List block lists authors alphabetically first vertically, then horizontally. This makes sense when listing pure text and when there's plenty of space between the columns, as in this example:

However, when the columns are more tightly packed and/or include headshots, a user may be inclined to read left-to-right, which would be a confusing order:

Describe the solution you'd like
The block could have an option on whether to display its alphabetical sort horizontally or vertically

Describe alternatives you've considered
Berkeleyside is manually building their author list without using this block.