Add interpolateName function and documentation

Extracted interpolation code from `file-loader` and enhanced it with regular expression support.
`[path]` is interpolated to the directory for the  `resourcePath`. If you provide a  context param, it'll be relative to this path. For the file-loader only relative paths make sense, but for the bundle-loader absolute paths should be possible too. So we should leave the `loaderContext.options.context` in the file-loader and allow to use absolute paths in `loader-utils`.