Make the --workers argument also available to the "burpa scan" command

[tristanlatr]

Currently, it's only part of the "burpa shedule" command.

The issue is that if one submit a list of 300 urls to the "burpa scan" command, 300 scans will be triggered right from the start, potentially using a lot of the Burp Suite Server resource.

It's probably not a big deal since Burp Suite should be able to handle such bulk requests, on the other hand, having the option on the "burpa scan" command is not a lot of extra work.