`noClear` does not work with multiple test modules

[cikasfm]

Hi!

Here's the scenario:

if I run a single test file with { noClear: true } - it works OK if I run a single test file with { noClear: false } ( or no option ) - it works OK but if I run mocha test/ where test/ is a folder containing 2 tests above - looks like mocha will still always run the clearDB on beforeEach and my #1 test will always fail.

Most likely it's due to the global variables in https://github.com/elliotf/mocha-mongoose/blob/be8a52ee947c25ad59bc8dee8a85777ad88cdf8b/index.js#L5:

var beforeEachRegistered = false;
var afterHookRegistered = false;

It would be good to "scope" these vars into the instance of clearDB