Support for new incident field

Hi there.

Incident.io have added support for an `external_issue_reference` field in the List/Show incident API call. Is there an intention to update the library to support this, or should I put together a PR? My intention would be to follow something like the `Severity` field in the `Incident` type, which is also a struct itself. From looking at the implementation of other fields, it should be as simple as adding the new Type for `ExternalIssueReference` and adding it into the `Incident` Type. Would anything else need doing?

Thanks!