Make this loadable easily by ZSH frameworks - #482

I don't know a whole lot about zsh but I support this effort.

Have you looked at the zsh stuff in the pro tips documentation? Is there any overlap there? If you want to move your stuff to the protips docs instead of the main README I'll land this code.

I'll update the doc there, then people can use dsf directly as a plugin without hassling with submodules. I think it's cleaner to have the framework support integrated in the tool repo.

Updated the doc per your suggestion & updated diff-so-fancy.plugin.zsh to conform better to the ZSH plugin standard.

Thanks!