Make `source-path` the first priority argument

[roydukkey]

Currently, the spec for the CLI is that the first optional, positional argument is backup-path. Then a source-path may optionally be specified before backup-path.

To simplify the API, source-path will be made the first optional, positional argument with the backup-path optionally following source-path.

[roydukkey]

The more I'm looking at this the more I'm content with the current API. Why? Because, though the project can be used on any old JSON file, it does imply usage with the package.json file. Therefore, the backup path is really more crucial.

Additionally, should clean-package configuration be cleaned from the source JSON file, it will longer exist at the source-path. In this case, requiring the source-path before specifying the backup-path makes no sense.

I'm gonna leave this open until the rest of the work is complete for v3 and reevaluate at that point.