Add force option

In response to https://github.com/gruntjs/grunt-contrib-qunit/pull/12 I redid this using the `force` option.

The use of `options` as a module-global is naff, but it's how the whole module-state works anyway. A better solution may be to migrate some of the code in https://github.com/sbrandwoo/grunt-qunit-junit as touched on in https://github.com/sbrandwoo/grunt-qunit-junit/issues/4

(This work is based off https://github.com/sbrandwoo/grunt-contrib-qunit/tree/fail-events)