Resolves #16

This PR adds support for parsing and managing multiple manifests. It migrates some directories as follows:

backups -> cargo
./ -> ships

So all the manifests including the core manifest which the tool uses for its main configuration will go under the ships/ folder. Any additional uniquely named manifests will also be parsed automatically.

On nostromo init, we'll try to migrate the files and folders by moving the core manifest and creating folders.
All other commands will now lookup using all the manifests collectively.

A couple of new fields on Manifest include the following:

// Name of the manifest or "manifest" if the core manifest
Name string `json:"name"`
// Source URL of the manifest which can be local or remote
Source string `json:"source"`
// Path of the manifest in local storage
Path string `json:"path"`