Add possibility to build classname based on string

I would really like the possibility to build a class name from a variable if the said variable is set. I encountered the issue when building classes for a standard Bootstrap alert box: in addition to the class alert I also needed the string alert-<alert-type>, but only if alert-type is defined.

[ojame]

We’re using ES6’s computed values (Babel, of course):

{
   [styles.Bar]: true,
   [this.props.className]: this.props.className,
}

[dcousens]

@ojame of course, @oleronning this is definitely the way to do this. Lets not over-complicate this module with what is essentially a language hitch.

Thanks @ojame