Fixup AttachmentFile::migrate()
Allow complex specification of arguments

Allow arguments to have sub-command 'options', so an argument like 'action'
can have a specified list of values, each with respective help output.
Add initial concept of file manager

Allows for backend listing, file listing, single file dump and file
migration.
Add backend to list output; add --limit switch

Add the current backend to the list results output
Add a --limit switch to support limit results sent to a command like
`migrate`
Merge pull request #432 from greezybacon/feature/cli-file-management

Add CLI app for file management

Reviewed-By: Peter Rotich <peter@osticket.com>