Fix and improve asset and collection export commands

[sjongejan]

Fixed several issues and improved flexibility for data import and export commands.

Import

• Added source_preset to the asset container export

Export

• Added support for --only-asset-containers and --only-assets flags in the assets export command
• Added support for --only-collections and --only-collection-trees flags in the collection export command
• Added support for --only-navs and --only-nav-trees flags in the collection export command
• Fixed bug in assets export
• Enabled independent exporting of collection trees. This allows collections to be maintained in static files while their trees are stored in the database (Eloquent).

[ryanmitchell]

Thanks for the PR and the work on it, all looks great.