Add support for customizing the name of the CSS modules property via the module attribute

[akanix42]

The default variable name (unchanged) is $style. Now, the user can control this, and even have separate modules, by setting the value of the module attribute (copying the code example from the updated documentation):

<style module="foo">
  .color {
    color: orange;
  }
</style>
<style module="bar">
  .color {
    color: purple;
  }
</style>

<template>
  <div :class="foo.color">Foo Text</div>
  <div :class="bar.color">Bar Text</div>
</template>

I think that's the last step to put this on par with vue-loader's CSS modules capability.

[Akryum]

Awesome! 😄