Add IID field to CreateIssueOptions

Add IID field to CreateIssueOptions, which allows explicitly setting
the internal ID assigned to the new issue at creation time.  This
allows preserving issue ID's when migrating issues into GitLab from a
different issue tracker.

See https://docs.gitlab.com/ce/api/issues.html#new-issue

issues.go

@@ -270,6 +270,7 @@ func (s *IssuesService) GetIssue(pid interface{}, issue int, options ...OptionFu
 //
 // GitLab API docs: https://docs.gitlab.com/ce/api/issues.html#new-issues
 type CreateIssueOptions struct {
+	IID                                *int       `url:"iid,omitempty" json:"iid,omitempty"`
 	Title                              *string    `url:"title,omitempty" json:"title,omitempty"`
 	Description                        *string    `url:"description,omitempty" json:"description,omitempty"`
 	Confidential                       *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`