Parallelize sass commands

Run as many sass commands in parallel as there are CPUs available.
Previously, each sass command would run serially, one after the other,
for each stylesheet that needs to be converted. This scales very
poorly for many stylesheets. On my 4-core, Intel i5 laptop, I was
seeing these numbers:

    Number files  Parallelized time (s) Serialized time (s)
    1             1.62                  1.65
    5             2.44                  3.46
    10            3.24                  5.68
    30            7.73                  14.69

As you can see, performance gains are seen immediately and there is no
consequence even when there is just one file.

The sass command line parameters don't allow processing multiple files
per run, which would presumably be even faster.