Use JSCS config loader by default with no specific file

By default, when the `config` option is `undefined`, or when it's `true`, the JSCS config loader will be used with no specific file passed in.
If `config` is a string, it will use the config loader only to parse it, as it used to be until now
If a falsy value is given, no config loading will be done and the config will be empty.

Also, this PR fixes some "conditional" unit tests.

Fixes #20
`null` is not undefined.