Provide a helper to create custom decorators

[ktsn]

This PR depends on #29, please see #29 before seeing this

This PR adds createDecorator helper to let us create custom decorators for class-style vue components. The helper expects a callback function as the 1st argument and the callback will receive a component options object. So, we can easily create Vue component-specific decorators.

Example:

import Component, { createDecorator } from 'vue-class-component'

export const NoCache = createDecorator((options, key) => {
  options.computed[key].cache = false
})

@Component
class MyComp extends Vue {
  // the computed property will not be cached
  @NoCache
  get random () {
    return Math.random()
  }
}

[yyx990803]

This is now conflicting after merging #29

[ktsn]

@yyx990803 I've resolved the conflict.