Improve the field loading method

[lionel-bijaoui]

This is a following to the discussion started in #232

The goal is to make field loading more modular, so that you only use the fields you really need.
@cristijora proposed this solution:

adding component names exporting fields in a separate file

//input field

export default{
name:'vfgFieldInput'
}

//fields index file

export default {Input, Checkbox} etc 

install(Vue,options) {
     Vue.component("VueFormGenerator", module.exports.component);
     if(option.fields){
       fields.forEach((field)=>{
         Vue.component(field.name,field);
       })
     }
}

Usage:

import VFG from 'vue-form-generator'
import {Input, Checkbox, AutoComplete} from 'vue-form-generator/fields'
Vue.use(VFG, [Input,Checkbox,Autocomplete])

This solution is a sweet spot between one big pack and only modular field. It allow to use internal component and custom ones, and take advantages of webpack tree-shaking feature.

Now, this also add some change to the plugin and some create some problem that we need to fix for v3.