More flexible Timeperiods

[johannesst]

@Nemental suggested this already for idoreports:
"Some hosts or services don't use the 24/7 period. An example would be 8/5, so from 9am to 5pm Mon-Fri. Incidents that don't happen during this period won't be considered in the calculation."

I think this should be added to the icingaweb2-module-reporting too.

[Nemental]

Icinga/icingaweb2-module-idoreports#31