support opn's options for loading a browser with arguments

[deanrather]

eg:

gulp.task('serve', function() {
    browserSync({
        files: [
            path.join('.tmp', config.contentFolder, '**/*')
        ],
        browser:
        {
            app: [
                'chromium-browser',
                '--app=http://localhost:8080',
                '--proxy-server=localhost:8080',
                '--user-data-dir=.tmp/chomium'
            ]
        },
        port: 8080,
});

[coveralls]

Coverage decreased (-0.1%) to 97.629% when pulling e38d667 on deanrather:master into 73596c9 on BrowserSync:master.