Add an ability to specify options

[vyorkin]

I've made this change to be able to specify custom defaults, don't know if you need this

[madeleineostoja]

At the moment options are defined in various CSS delcs, since you'd probably want to change them on a rule-by-rule basis. But I could imagine valid use-cases for the ability to set globals via the js API like this.

Would need a more thorough PR, with tests and doc changes. I don't have time to do that atm, feel free to add to this and I'll accept it though.

[vyorkin]

Sorry, I didn't have much time to do this, please keep it open for some time (1-2 weeks maybe :)), I think I'll manage to do something about it

[vyorkin]

still don't have time, but I remember about this :)