Self-publish can't replace packages

[ohanhi]

It would be great if elm_self_publish.py could replace an existing package by the same name and version. This seems like the usual case when developing an Elm package: self-publish the package under development, make some changes, self-publish again.

Another thing is that with Elm Native UI we need to replace the official core with a modified one, so the workflow now is the following:

1. elm-make the first time in the project, which creates an elm-package.json and elm-stuff with the official core
2. Remove official core by hand
3. Self-publish the modified core
4. Self-publish elm-native-ui

We could at least get rid of step 2 with this change. :)