Hello!

See two attached patches.  All tests pass on my computer.

Every once in a while I feel obligated to go back to org-clock-sum to
try and optimize it.  I have a file with 8 clocktables in it and it
takes forever to update.  This time I decided instead of trying to
optimize, I'm just going to try and understand.

The regex has been altered slightly.

1. Instead of using "[ \t]", I decided to use [[:blank:]].  No real
reason.  I just think it's easier to read and maybe slightly more
correct?

2. For the timestamps, instead of ".*?" (using a non-greedy ".*") I
decided to use "[^]]*" (accept everything except "]").  I did this simply
because I'm not used to using non-greedy regex's.  Maybe this way
performs better?  I didn't test that.

3. I used the variable `org-outline-regexp' but that doesn't actually
change the regex.