Allow to skip steps in clock reports that have zero time

Allow to skip steps in clock reports that have zero time

* org-clock.el (org-dblock-write:clocktable): Return total time.
(org-clocktable-steps): Skip step when time is zero and the
:stepskip0 property is set.

Rainer Stengele writes:

> Creating clocktables for each day of a month is an excellent feature
> in org!  Problem for me is that many resulting tables have a total
> time of 0:00.  The problem is not really the zero time I spent but the
> appearance of the tables with zero total time.
>
> Is there a possibility to skip getting such tables?  Rational: I do
> not need to show my boss or customer days where I spent 0:00 time on
> the project.

Now you can set :stepskip0 to achieve this.