Add simtime option

[smihael]

According to http://wiki.ros.org/rosconsole#Console_Output_Formatting
rosconsole allows you to specify how you'd like its output to show up in the console output through the ROSCONSOLE_FORMAT environment variable.

By default timeshows the wall time first and the sim time second if sim time is available it shows the wall time first and the sim time second.

There is separate entry for walltime, but no entry for simtime only.

This would be useful to have differently formatted output for walltime and simtime.

      <env name="ROSCONSOLE_FORMAT" value="[${severity}/${node}] [${walltime:%Y-%m-%d %H:%M:%S}, ${simtime %H:%M:%S}]: ${message}"/>