Add a parallel option to go-zopfli cli tool

Parallel operation is easy for gzip format, because the
format itself supports concatenation. Thus we create one
goroutine for each 1/n of the input byte slice and concatenate
the compressed streams into the output file.

Parallelization is not yet supported for other file formats.
Merge pull request #1 from jeffallen/master

Add a parallel option to go-zopfli cli tool