feat(logcli): query from absolute timestamp (#736)

* feat(logcli): query from absolute timestamp

Adds a new flag (`--from`) as to complement `--since`.
While since subtracts a relative duration from the current time, from allows to
specify the absolute start of the lookback window.

Note: `--from` takes precedence over `--since`, but only if set.

* fix(logcli): use RFC3339Nano for -from

To comply with Prometheus, this flag now honors Nanoseconds as well (was using
RFC3339 so far)