New: add "no-unused-vars"

[g-plane]

Fixes #33 .

[aladdin-add]

it is required to use {ecmaVersion: 6} in eslint v6 (not published yet).

also, you can use const ruleTester = new RuleTester({parserOpitons: {...}});

[aladdin-add]

the code looks good to me.

can you add some more tests like:

var undef; // => autofix
var t = `foo-${bar()}`; // no-autofix
var a = b = c; // either

[aladdin-add]

LGTM, thanks!