When customSelectors is empty, console gets polluted

[michaelchance]

Using cssnext, which uses this as a dependency. I noticed that in some cases, cssnext doesn't actually use this module, but the log gets polluted with false positives, such as:

"Warning: The selector '.ui-widget-overlay' is undefined in CSS!"

In addition to the check for if(flag === 0) maybe it should check if customSelectors is empty?

[MoOx]

I will push a fix asap.

[MoOx]

I pushed a PR #23

I am not merging it since I am not the maintainer and don't have the right to publish (yet? poke @yisibl).