Fix vertical legend alignment for --legend-direction=bottomup

Issue 419
Add a --legend-direction=bottomup variant

The --legend-direction=bottomup2 variant will exempt leading and
trailing COMMENT lines from re-ordering. This might be useful for
generators like munin that use such lines for table headers and the
like.

Issue 419
Merge pull request #422 from cbiedl/master

Fix vertical legend alignment, enhance --legend-direction