cbbackup: Unknown type of source with HTTPS

If one attempts cbbackup using the -s option and specifies HTTPS protocol for the target URL, an error message is shown.

Example:

```
cbbackup -u Administrator -p password -s https://brian3.local:18091 /opt/cb_backups/
```

Results in:

```
error: unknown type of source: https://brian3.local:18091
```

It is perhaps counterintuitive and confusing to perform secure backups with -s or --ssl option to a plain HTTP protocol based URL target. Can we please have support for HTTPS source and updated the cbbackup documentation accordingly?