Upgrade eslint; Add `--es6` flag; Add `--line-length` flag

[andrewdeandrade]

This pull request upgrades the version of eslint used and adds an --es6 flag (off by default) and a --line-length flag that you can use to set the line length.

Once everything is good, I'll merge this in as major version change.

The only new rule I was uncertain about is the no-continue, which I think would be good to enable. https://github.com/eslint/eslint/blob/master/docs/rules/no-continue.md

I also was curious if anyone had any opinions on turning on the object-shorthand rule for the --es6 flag? https://github.com/eslint/eslint/blob/master/docs/rules/object-shorthand.md

@raynos @lxe @mlmorg

[freeqaz]

I agree with both of those rules. Better to disallow continue now before people depend on it.

That object-shorthand rule is neat.

[freeqaz]

lgtm