Adds lint and test pre-commit hooks

[oneill-c]

It occurred to me while reading the contribution guide that the project might benefit from automatic push hooks to lint and test.

[coveralls]

Coverage remained the same at 92.835% when pulling a481154 on mcpolandc:add-pre-push-hooks into a252668 on JedWatson:master.

[JedWatson]

@mcpolandc thanks! good idea.

Thoughts on using husky and lint-staged with a pre-commit hook though? (instead of pre-push, errors would be caught before commit)

[oneill-c]

Hi @JedWatson! Yeah, running the linter on only the staged files when committing makes a lot of sense. I will make changes to this effect 😄

Just extending on this... I wonder if we should maybe add the test cover check at the commit stage as well. Maybe husky, lint-staged or some other package can provide this functionality (exit with 1 if test coverage has dropped, thus failing commit). What are your thoughts on this?

[oneill-c]

Changed this to run lint (only staged js files) and test scripts on commit rather than push.

[coveralls]

Coverage remained the same at 92.868% when pulling 207ca7c on mcpolandc:add-pre-push-hooks into ca09183 on JedWatson:master.

[JedWatson]

What is the point of git add here? unless eslint is being run with the --fix flag (which it's not and I'm happy to leave it that way) this wouldn't be necessary; and it has the side-effect of preventing you from committing only staged chunks of a file without making it an obvious thing...

Surely just eslint on its own would be sufficient (and safer!) here?

[oneill-c]

I knew that I didn't want to have it fix linter errors automatically but I didn't really understand what git add was doing. Reading further down the readme of lint-staged helped with that 😛. I totally agree with what you're saying here so making the change.

[JedWatson]

Thanks @mcpolandc! Left a note there for feedback but otherwise this looks great!

[coveralls]

Coverage remained the same at 92.9% when pulling cdedc32 on mcpolandc:add-pre-push-hooks into f057218 on JedWatson:master.

[JedWatson]

Great, thanks!