Add interpolateName function and documentation

Extracted interpolation code from `file-loader` and enhanced it with regular expression support.
Could you move this to the parameters? So a loader could use it for other query options too or provide it in any other way. Could you move the `content` parameter into an `options` parameter object, so it can be extended later.

``` js
function interpolateName(loaderContext, name, options) {
var content = options.content;
```