Allow empty prefix

[cvle]

Setting to an empty string still results in the default prefix.
I was expecting it to disable prefix completely.

[wooorm]

Hi, @cvle! Thanks for raising this.

This is mainly a port of highlight.js. Do you know if that has the same problem, or is there a difference in behaviour between these?

[cvle]

I am not familiar with hightlight.js so I don't know. I mainly use your library (and it works great) ;-)

[wooorm]

Great, thanks! But this is a fork, so I’ll only do what they do (albeit virtualised), so if you want it here it needs to be in highlight.js too.

[wooorm]

I just checked with highlight.js and as they have global configuration, and lowlight differs in that. I’m OK with doing this!