make CampaignCreationSegmentOptions fields omitempty

[njason]

recipients.segment_opts in MailChimp API docs is optional, hence I have modified the CampaignCreationSegmentOptions struct accordingly.

I did not like the solution in #53 and #60 which makes SegmentOptions a pointer because it is breaking change. However that is the only way to use the omitempty tag on a struct field.

My solution is to instead place omitempty on all the fields within the struct. When the CampaignCreationRecipients is JSON marshalled and SegmentOptions is not provided, it will generate "segment_opts": {} which behaves the same at it being omitted in the MailChimp API. This solution also does not break existing usage.

[njason]

@zeekay @bradleyfalk @tw1nk @Rishub21

Can someone please review?

[zeekay]

Looks good, thanks for your contribution!