Add setting to defined custom Default Interval

[josiah47]

Would be nice to have this since we care about total hours for current week not including last weeks.
example:
If its Friday, and I am reviewing my total hours this week to see if it matches my time card total in another system. It will by default include last weeks Saturday overtime hours because of the 7 day default.

for now I just hack the app/controllers/spent_time_controller.rb file and change 7_days to current_week
I do not have much knowledge of creating the settings file but maybe it can be done as part of #119