Add support for custom resolve-url-loader join function

[jclausen]

In the previous version of elixir, URLs in SCSS files which could not be resolved were ignored. Current version of sass-loader are more strict about being able to locate the asset. There are workarounds ( discussed in PR #42 ), however the correct method of handling these on an application-by-application basis is to use the custom join closure functionality in the current version of resolve-url-loader.

Add the ability to provide a custom join closure to the Elixir config, which will be sourced in to the resolve-url-loader config at the time SCSS files are mixed.

Example:

elixir.urlResolver = ( { isAbsolute, bases: { substring, value, property, selector} }, { root }) 
                                     =>  isAbsolute ? [root] : [subString, value, property, selector]