Support `data` option in preprocess style options to share common content as vars or imports

[tonimc]

Allow prepend a common style definition in the component which will compile, as you can do using a style-loader in webpack.
For example, it's useful for scss be able to define a set of variables or imports and avoid writing in each component the same lines, as is supported with webpack on vue-loader https://vue-loader.vuejs.org/guide/pre-processors.html#sharing-global-variables