Support scoped css

[ktsn]

Changes proposed in this pull request:

• Support scoped css
  • Inject scope id in component options object in compile time so that all elements in a component has data-v-xxx attribute.
  • Transform css selectors to have scope id. It uses the same logic with vue-loader and vueify.
• Small refactoring to avoid eslint warning 😛
  • Split processScript function to reduce complexity.

I'm not sure about docs. Should we add 2.4 docs and add about scoped css feature in it?

/ping @znck

[znck]

I'm not sure about docs. Should we add 2.4 docs and add about scoped css feature in it?

I think updating 2.3 docs with a warning tip would work.

[ktsn]

@znck Updated the docs 🙂

Also I just notice the injected code is missing trailing comma. I've fixed it too.