gulp-sourcemaps  support

Trying to support [gulp-sourcemaps](https://github.com/floridoo/gulp-sourcemaps) to deal with multi-stage sourcemaps under gulp.

ref: [Plugins with gulp sourcemaps support](https://github.com/floridoo/gulp-sourcemaps/wiki/Plugins-with-gulp-sourcemaps-support)

- [x] add vinyl-sourcemaps-apply to dependencies
- [x] add multi-stage-sourcemap to dependencies
- [x] use espower directly
- [x] maint tests
- [x] functional tests
- [x] README