logcli query --since: support absolute times

[srstsavage]

It would be nice if logcli query's --since option supported absolute dates as well as relative durations. For example, using an ISO 8601 time string:

logcli query --since=2019-07-08T14:53Z -t '{label="somevalue"}'

or epoch:

logcli query --since=1562622455 -t '{label="somevalue"}'

Could use a different option name if it's too difficult to parse relative durations and absolute times using the same option.