Add --concurrency flag

[jopemachine]

Fixes #21.

I'm not sure I need to write test codes about this flag, so I tried to simply benchmark the option using hyperfine.

os.cpus().length on my local computer is 16.

Here are the benchmark results.

$ hyperfine --runs 10 'node ./cli.js ./node_modules ./abc_node_modules' > result1

Benchmark 1: node ./cli.js ./node_modules ./abc_node_modules
  Time (mean ± σ):      5.029 s ±  0.919 s    [User: 5.521 s, System: 5.258 s]
  Range (min … max):    3.665 s …  6.315 s    10 runs

$ hyperfine --runs 10 'node ./cli.js ./node_modules ./abc_node_modules --concurrency=4' > result2

Benchmark 1: node ./cli.js ./node_modules ./abc_node_modules --concurrency=4
  Time (mean ± σ):      5.739 s ±  1.223 s    [User: 6.431 s, System: 5.027 s]
  Range (min … max):    4.451 s …  7.475 s    10 runs

If I need to do other things, I appreciate your letting me know.

[sindresorhus]

Looks good. Thanks :)