It's somewhat common to use relative paths in .css files.

Versioning such files with elixir invalidates all relative paths:
http://stackoverflow.com/questions/27789466/asset-management-maintaining-reference-to-relative-assets-after-concatenation/32223524#32223524

This PR proposes a simple syntax to solve this kind of problem, by allowing users to define an array of paths relative to **/public** as a second (or third) argument, so they'll be copied to **/build** after `version` command executes.

**Usage**
```
mix.version(
    ['css/style.css', 'css/vendor/style.css'], //files to be versioned
    ['fonts', 'css/vendor/icons'] //dependent files/dirs to be copied
);
```

This will also re-copy every dependent files when using `gulp watch`.

There was a refactor from @JeffreyWay in this file on main repository before my PR was accepted.

So, I solved the resulting conflicts and developed a cleaner version of the proposed feature, based on his last commit.

Also, I removed the option for ommiting the second argument. While slightly handy for the end user, the implementation was not nice code.