Allow field formatter to work with list_text as well

[herbdool]

In my use case I want to preset a bunch of icons for a field, and not leave it up to the author to know what classes mean.

This is easily done with the list_text field type. I've got it working so I can provide a pr soon.

By the way, it wasn't clear at first where this formatter would be available and how one should use it. I had to look in the code. (Oops, I missed that part of the readme).

[herbdool]

@BWPanda I've got a PR.

[ghost]

Thanks @herbdool! I made a small change to fix the line length in the README, but otherwise merged your PR as-is.