index.js: Added ability to change output extension

Is to rename the "handleExtension" by "handleClient" function
maintaining it's functionality.

And in the new "handleExtension" function will be replaced the output
extension.

Example of use:

gulp.task('jade', function() {
    return gulp.src(path.src.jade)
        .pipe(jade({
            ext: '.phtml'
        }))
        .pipe(gulp.dest(path.dest.jade))
});