add '-exclude' list to exclude specific zones

[jschauma]

When running 'redownload', a user may wish to exclude some zones. To do this, you can now specify the '-exclude' option. This option takes as arguments a list of comma-separated zone names to avoid downloading.

'-exclude' cannot be specified together with '-zone'

Example:

czds-dl [...] -exclude com,net,biz

[lanrat]

Thanks for this feature!

Can you update README.md with the new flag information?

Using the output of the command help is enough.

[jschauma]

Done.