add a cp subcommand with scp/rsync/cp behaviour

[torwag]

Hi, I always struggle with the way rmapi takes it parameters. I believe my thinking comes along the ideas reported in #134. For me most natural way to copy over files to and from the remarkable would be to use a rmapi cp -r syntax. Whereas -r indicates recursive behaviour. Other flags could deal with the generation of annotated PDFs or how to deal with existing files. If some new sub-command gets used for this. rmapi would still be backward compatible. To not further increase development effort, by time the old commands put and get could be simply an alias for a specific cp command. The reason I prefer cp over sync would be that sync might be misleadingly understood by users to do a two-way sync operation.