chore(package): add lint-staged eslint autofix

[JLHwung]

This PR introduces pre-commit hook to run eslint autofix on staged .js files. Most of time you will not waste your time adding whitespace or commas after you draft a PR and the travis fails due to style issues. No more, it is frustrating and tedious. We really treasure your time.

In this time you simply draft your ideas and let eslint fix most style issues for you on commit, you just sit and don't have to run eslint after you revise files.

git-exec-and-restage is less popular but it proves pretty handy over half a year coporate trial. Basically git-exec-and-restage will only run git add on fully staged file. It runs linter on partially staged file and it depends on you to commit the style changes of specific hunks later.

[coveralls]

Coverage remained the same at 97.249% when pulling dae3bbc on precommit-eslint into f19629e on master.

[coveralls]

Coverage remained the same at 97.249% when pulling dae3bbc on precommit-eslint into f19629e on master.