Timestamp formatting options

[jalvz]

Add flags for an alternative (aka shorter) timestamp format, and to convert to local time zone.

[trentm]

For the record, some earlier work and discussion at #14

[trentm]

Some possible time formats:

• 2021-05-20T22:50:49.878Z default format is "rfc3339"
• 22:50:49 or 22:50:49.878 - "time", just the time (with different precision)
• 2021-05-20 15:50:49.878 - "local", "local datetime", at least for me (though to be RFC compliant it should include the time offset, but that's long). Could drop the "T" to indicate that we are off spec.
• 15:50:49.878 - "local time"
• 21-05-10 13:40:04 - jalvz short form from Space time #14, not sure about a name for this one.
• I'm hesitant to support the Go time format because it so weird and awkward.
• 49.878 - "seconds" or "s" for short
• 50:49.878 - "minutes" or "m" for short
• 15:50:49.878 - "hours" or "h" for short
• 0, 1h2m0.3s - "offset" (or some other name) to show the duration since the
  first record, using 0 for the first record. Using go's func (Duration) String.
• Perhaps another one to show the duration since the last record, rather than
  the first. What name for this one?

Proposal: Implement a -t, --time format (timeFormat config var) option for the format to use for the @timestamp, implementing some or all of the names in the list above.

Q: What about whether to display brackets around the time?

Some title lines from demo.log, apm-server, elasticsearch, and kibana:

[2021-02-11T06:24:53.251Z]  WARN (on purple.local): error fields
[2021-05-20T22:53:45.304Z]  INFO (request): request accepted
[2021-05-20T22:50:49.878Z]  INFO (org.elasticsearch.cluster.metadata.MetadataCreateIndexService/ES_ECS): [.kibana-event-log-8.0.0-snapshot-000001] creating index, cause [api], templates [.kibana-event-log-8.0.0-template, .kibana-event-log-8.0.0-snapshot-template], shards [1]/[1]
[2021-05-20T22:50:50+00:00] : Installed all assets

For starters, time format conversion should assume the raw @timestamp
is in RFC3339 format.

[trentm]

Actually the default format should be "raw" or something, i.e. use the string exactly as it is in the log record.