title: luxon convert local time to utc given a timezone
body: <p>A data source has an ISO-8601 datetime field without an offset.</p>
  <p>Example: <code>2019-07-09T18:45</code></p>
  <p>However, I know that the time in question is understood to be in the <code>America/Chicago</code> timezone.</p>
  <p>How can I get a Luxon DateTime object of this time's equivalent in UTC?</p>
  <p>I can do  <code>DateTime.fromISO('2019-07-09T18:45').plus({hours: 5})</code> ... but this will only be valid during half of the year (like now) when it is daylight savings time. Otherwise the offset would be <code>.plus({hours: 6})</code></p>
  <p>Does Luxon have a date-aware (and therefore DST-aware) method for converting from a specific zoned local time to UTC?</p>