make file inspection parallel

@bbatsov
optionally run in parallel (does not support progressbar formatter since file is processed in a new process)
I also tried threads but did not see any speedup

```
[~/Code/tools/rubocop (grosser/parallel2)☃] ➔ time be ./bin/rubocop spec/**/*
Inspecting 207 files
...............................................................................................................................................................................................................

207 files inspected, no offenses detected

real	0m16.797s
user	0m16.380s
sys	0m0.408s
[~/Code/tools/rubocop (grosser/parallel2)☃] ➔ time be ./bin/rubocop spec/**/* -p
Inspecting 207 files
...............................................................................................................................................................................................................

207 files inspected, no offenses detected

real	0m10.103s
user	0m29.090s
sys	0m1.078s
[~/Code/tools/rubocop (grosser/parallel2)☃] ➔
```