Remove ES6 transpiling and add <style> support

[thgh]

I hope this PR doesn't diverge too much from what you had in mind for this plugin. The transpiling has been removed completely, which can now be done as a separate plugin (see below).

Some notes:

• Unwanted Treating '...' as external dependency even when using node-resolve & commonjs.
• Should use ongenerate instead of banner
• Might have removed too much Promises.
• Currently when using multiple style->lang values, each value gets a file. Can be optimized. To be discussed in How to rollup Vue component with styles? #7
• Should check compatibility with the babel plugin Please remove babel dependency #8

Example rollup.config.js

import vue from 'rollup-plugin-vue'
import buble from 'rollup-plugin-buble'

export default {
  entry: 'src/main.js',
  dest: 'dist/bundle.js',
  plugins: [
    vue(),
    buble()
  ]
}

[znck]

This looks cool.

[znck]

@thgh I guess I have applied your commits. Please sync your fork.