Adds a tar action for generating a tarball of release and support for mulitple actions

Multiple actions can now be specificed for a `relx` run. Example, `$ relx release tar`, this will generate the release and then a tarball of the release.

With this change it is now the case that `$ relx relup` will not generate a release first. You must run `$ relx release relup` in order to both generate the release and then the relup, if you want to do it all at once.