[TI-54] Support for datetime format

_Maarten on 2016-11-18T09:10:45Z says:_

It is currently impossible to ommit seconds in the output for timewarrior reports. For reporting/readability; seconds are somewhat too much of a good thing for management. Ffor time registration itself, it is fine.

It would be nice to be able to set something like:

rc.report.summary.dateformat = Y-M-D hh:mm

or

dateformat.report = hh:mm