[jss-namespace] remove option "named", introduce a plugin instead.

[kof]

Problems it solves:

• ambiguity of keys, with named: true it is rule names, with named: false it is selectors
• SSR requires a different class name generation algorithm

JSS without jss-namespace plugin generates global selectors.
Once plugin is used, class names are generated using counter, like it is right now + user can specify a function which can use for e.g. murmurhash to generate an id. This will make it predictable and enable rehydration. #221

[kof]

In this conversation https://github.com/styled-components/comparison/pull/33/files#r80881006 I have got a different idea.

What if we move global styles to a separate plugin?
We can make then an option {global} as well as json key global, which will allow to create global styles right within local sheet.

{
  global: {
    html: {globalStyles},
    button: {globalStyles}
  },
  button: {localStyles}
}

[kof]

Closing this issue in favor of #324