Don't use `parse_known_args`

We *do* know which scalegrease commands that should take additional
arguments and which ones doesn't, so I moved it into
extra_arguments_adder, where it is needed.
Use luigi --task to qualify log file paths

Previously, multiple jobs in one jar will create colliding log paths
when executed from Scalegrease. Usually that is the intention, but it is
nice for each luigi task to have its own log file.
Merge pull request #20 from Tarrasch/colliding-log-paths

Colliding log paths