Backward compatible version of #62 -- This version does *not* change the
default behavior. Users must opt in to new behavior.

This patch updates the file sink to allow *not* appending the timestamp
on the current log file. In this mode the timestamp is only appended
when the file is rotated.

This matches the behavior:

- Implemented in Nomad v1.1.4 with hashicorp/nomad#11070
- Requested in hashicorp/nomad#11061

> The following example configures a destination called "My Sink" which
> stores audit events at the file `/tmp/audit.json`.

Nomad's documentation is a little vague but implies the prior behavior
of always appending a timestamp.

While Go compares time using a monotonic clock that only prevents time from
moving *backward,* it does *not* guarantee time will move forward
between subsequent calls to `time.Now()`. The After assertion failed in
a test run on Github because it is missing the `Equal` comparison on the
preceeding assertion.