Add isort to pre-commit - #379

what do we check here?

Checking if unordered imports aren't remained.
python -m isort -rc . also outputs diff when unordered imports remained.
To cover the case that developers don't set pre-commit and uploaded files have unordered imports. (Same to autopep8's check)

shouldn't we check the exit code instead ($?) ?

Your comment is exact.
I've checked its behavior at first, but unfortunately both cases(ordered, unordered) return 0.

how about https://stackoverflow.com/questions/2480584/how-do-i-use-a-file-grep-comparison-inside-a-bash-if-else-statement ?

Will look into again.
At least I found below issue(fixed) and codes. Sholud be returned non 0.

PyCQA/isort#423
https://github.com/timothycrosley/isort/blob/d093f97c1d26854fbb082e43cfae8a062dacda56/isort/main.py#L382

I could do as expected by using --check-only opt for isort :)