Add support for negative prefixes in zIndex plugin

[adamwathan]

This PR makes it possible to create classes like -z-10 using the same convention as the inset and margin plugins where config keys beginning with a - translate to classes beginning with a dash:

Example:

zIndex: {
  '-10': '-10',
}

The above config will create a class called -z-10, not z--10 like you might otherwise expect.