Emit "load" and "error" events on loading

[vgavro]

For usage like this:

<template>
<script2 src="myscript.js" @load="loaded" />
<template>
<script>
export default {
   methods: {
       loaded () {
          window.myscriptModule.youVeGotTheIdea()
       }
   }
}
</script>

fixes with #29

[taoeffect]

Note: the load event was renamed to loaded. Available in 2.1.0!