Add ability to change list-item width from the list-item block

[aurooba]

After chatting with Matt, I'm thinking it will make more sense to be able to change the size of the list-items from the list-item block settings itself. With a note that this affects all list-items.

[aurooba]

Thinking through @justintadlock's feedback. What if it was something like a column control with a slider BUT also a little custom button so you can set your own specific size in units yourself?

And thinking further out, is this block a candidate for more complex layout-design? Or would that dilute the plugin? Should that be in its own plugin?