cli: allow copy startup-config my-backup-config

[troglobit]

Currently the CLI is limited to common source and target configs: running, startup, and factory, with some limitations.

The copy command should allow a user to copy to any location and name. E.g.,

copy startup-config /usb/my-backup.cfg
copy startup-config tftp://1.2.3.4/my-backup.cfg

and

copy /usb/my-backup.cfg running-confg

Note: this is just an example, the critical feature described here is any name. Supporting various URIs or even different storage is not in the scope, only worth considering when planning the implementation.

[troglobit]

Extensive support, including for remote files using curl, in PR #231