Support full syntax in function rules and values

[kof]

Currently the only plugin that we run over function values/rules is jss-default-unit. The problem with running every plugin over it is that we would add performance overhead to something that is supposed to be usable for animations.

Originates in https://github.com/cssinjs/react-jss/issues/185

Cases:

• fallbacks in fn rules
• media queries inside of function rules
• media query as a fn rule nested
• function values inside of media rules
• jss-nested inside of fn rules
• jss-nested as a fn rule
• jss-global should support function values and rules
• jss-compose should support linking function rules
• jss-compose should support linking rules with fn values
• jss.use(pluginObservable({process: false})), test for disabling plugins

[kof]

I got an idea what we can do here: due to the fact that other cssinjs libs have also functions and they don't expect those to be updatable at 60 FPS we can do this:

1. We assume that mainstream use case doesn't need to update at 60FPS
2. We allow all required plugins to react on the values/rules changes like they do for normal values
3. Introduce a modified interface for "fast" functions:

{
  button: {
   // If value is an array and first item is a function, we got a function value.
    top: [props => props.top]
  }
}

{
   // If rule is an array and first item is a function, we got a fast function rule.
  button: [(props) => ({
    top: props.top
  })
}

Additionally we could allow passing options:

{
  button: {
   // Second array value is an options object.
    top: [props => props.top, {someOptions}]
  }
}

Another interface idea:

// This function name could be something else…
import {fast} from 'jss'
{
  // fast() will mark the function with a flag that allows to distinguish "fast" functions without plugins.
  button: fast((props) => ({
    top: props.top
  }))  
}    

I am open for any other interface suggestions.