Support for a single transform

[caseywatts]

I'd love to be able to use codemod-cli without restructuring my project into the multiple-transforms format.

I made a codemod recently, and it only has one transform. codemod-cli has a lot of features I really want to use, but I set the project up before discovering codemod-cli. At that point it was hard to switch over to the filesystem structure that codemod-cli expects. It also feels "wrong" to restructure folders to be nested to make room for multiple transforms when I know this project was never going to have more than one.

This would help with migration path to adopting codemod-cli for existing codemods. It might also help with on-boarding, if the single transform setup ends up with less to wrap your head around.

@rwjblue and I were talking about this on Twitter some :)