Add noglobal support.

Allow a `noGlobal` flag to be specified in config, causing a test to fail if it pollutes the global namespace.

QUnit has existing support for this feature, triggered by the presence of a query string param called `noglobal`.
Correct header level for `noGlobal` section in README.
Correct `noGlobals` param info formatting in README and options docs.
Merge pull request #77 from longdivision/add-noglobal-support

Add noglobal support.