syslog: Add support for SYSLOG_TIMESTAMP (#260)
syslog: Add support for SYSLOG_TIMESTAMP (#260)

Logspout is currently hardcoded to use RFC3339 timestamps. This can
cause problems when using SYSLOG_FORMAT=rfc3164 because some
old-fashioned syslog servers insist on "Mmm dd hh:mm:ss" format.

Make logspout more flexible by adding support for a SYSLOG_TIMESTAMP
environment variable containing a Go template. This template will be
applied to a SyslogMessage value and defaults to {{.Timestamp}}, so
there is no change to the current behavior.

Legacy "Mmm dd hh:mm:ss" timestamps can now be specified with:

    SYSLOG_TIMESTAMP='{{.Time.Format "Jan 02 15:04:05"}}'

Or RFC3339 timestamps with nanosecond precision can be specified with:

    SYSLOG_TIMESTAMP='{{.Time.Format "2006-01-02T15:04:05.999999999Z07:00"}}'
bn0ir/logspout,gliderlabs/logspout,bn0ir/logspout,jacobdr/logspout,jacobdr/logspout,lytics/logspout,lytics/logspout,gliderlabs/logspout