[TI-38] Time budget support

_Yury V. Zaytsev on 2016-08-23T17:32:18Z says:_

It would be awesome if Timewarrior supported time budgets / accounts.

One should be able set the available budget for a specific tag e.g. to 8 hours per day every workday. The balance will then be automatically increased by 8 hours every workday and decreased by the time logged to this tag. Summary reports should show the current balance & logged time for the tag.

It is also important to be able to define holidays / vacations, etc. so that on these days, the balance doesn't auto-increment. It would be nice to be able to set starting and ending time points for the auto-increment function.