Make all elements of IssueOptions be Options

[Hrothen]

This is a breaking change, should this PR also update the project version?

[dwijnand]

Hmm, this PR is quite old and a little bit stale too (syntex is no longer used and filenames have changed).

Also, I wonder if the objective here (making issue title and labels optional) could be approximated without breaking the API, i.e with an empty string and an empty Vec (which I don't think carry a distinct meaning).

@Hrothen how interested are you in this change landing?

[Hrothen]

Not at all, this PR is 2 and a half years old, I've moved on.