Support of Custom Elements

This release fixes #5 and add support of custom elements #4

Usage
Use FormSchema.setComponent(type, component[, props = {}]) to define custom element to use for rendering.
See vue-json-schema-demo-elementui for a complete example.

// an element-ui example

import FormSchema from 'vue-json-schema'
import {
  Form,
  FormItem,
  Input,
  Radio,
  Checkbox,
  Select,
  Option,
  Button
} from 'element-ui'

FormSchema.setComponent('label', FormItem)
FormSchema.setComponent('email', Input)
FormSchema.setComponent('text', Input)
FormSchema.setComponent('textarea', Input)
FormSchema.setComponent('checkbox', Checkbox)
FormSchema.setComponent('radio', Radio)
FormSchema.setComponent('select', Select)
FormSchema.setComponent('option', Option)

// Use the third argument to define props of the component
FormSchema.setComponent('button', Button, {
  type: 'primary',
  label: 'Subscribe'
})

// The third argument can also be a function that return an object
FormSchema.setComponent('form', Form, (vm) => {
  // vm is the FormSchema VM

  const labelWidth = '120px'
  const model = vm.data
  const rules = {}

  vm.fields.forEach((field) => {
    rules[field.name] = {
      required: field.required,
      message: field.title
    }
  })

  return { labelWidth, rules, model }
})

export default {
  data: () => ({
    schema: {...}
  }),
  // ...
  components: { FormSchema }
}