support regular expressions in opts.extensions

I'd like to only inline certain images, and by adding support for RegExp in `opts.extensions` I'm able to name my image files `myimage--datauri.png`, and configure my gulp pipe like so:

    .pipe( base64({
        baseDir: 'dist',
        extensions: [ (/--datauri\.(?:svg|png|gif|jpg)(?:$|\?)/) ]
      }) )