Add report on time spent on projects

[lauft]

Timewarrior allows for multitasking via tags. You can assign multiple tags to a single interval, e.g.

timew start MONITORING PROJECT1 # monitoring all day, starting with project 1
timew start MONITORING PROJECT1 PROJECT2 # adding project 2 to the pool
timew stop PROJECT1 # done with project 1, still monitoring and working at project 2
timew start MONITORING PROJECT2 PROJECT3 # adding project 3
... # something similar with PROJECT4 and PROJECT5
timew stop # done for today

This creates a consecutive chain of non-overlapping intervals in the database, keeping it thus sane and clean.

Create a report, which fuses/filters your simultaneous work into something like

timew my-report
MONITORING 8h
PROJECT1 4h
PROJECT2 2h
PROJECT3 5h
...

[lauft]

Spawned from #151