showlog: Add support for --time-format=duration
This is immensely useful when debugging report-bugs and other
types of logfile- or report anomalies. Calculating the duration
of each event used to be an immensely tedious task, and
especially so when we get large report-bug-reports (ehh...),
which can contain several million lines.
So far, this is really only useful for calculating totals for
a single object, but with the groundwork laid we can easily
build a per-object state duration counter for this feature.
Signed-off-by: Andreas Ericsson <ae@op5.se>