Diff dotfiles

It would be useful to be able to diff all dotfiles at once:

```
rails-diff dotfiles
```

I'm considering dotfiles the files outputed by:

```sh
find . -path ./.git -prune -o \( -path './.*' -type f -print -o -path './.*/*' -type f -print \)
```

_and_ is not ignored by git

That is: a dotfile in the toplevel dir and files inside toplevel dirs that start with a dot like `.github/`.