Avoid sharing _options across multiple streams (#69)

Before this commit, variable _options is sharing multiple streams.
It causes problems. _options.rebaseTo that was always set first is used.

If we have below files and gulpfile.js, rebaseTo was always `<PROJECT_ROOT>/scss/entries/lp/lp1/css`, and never `<PROJECT_ROOT/scss/entries/lp/lp2/subpage/css>`

```
scss/entries/lp/lp1/css/index.scss
scss/entries/lp/lp2/subpage/css/index.scss

gulp
  .src('scss/entries/**/*.scss')
  .pipe(plumber())
  .pipe(sass())
  .pipe(cleanCSS()) // rebaseTo always 'scss/entries/apps/app1/css/',
  .pipe(gulp.dest('public/'));
```

index.js

@@ -6,11 +6,12 @@ const through = require('through2');
 
 module.exports = (options, callback) => {
 
-  let _options = Object.assign({}, options || {});
   let _callback = callback || (o => undefined);
 
   return through.obj(function (file, enc, cb) {
 
+    let _options = Object.assign({}, options || {});
+
     if (file.isNull()) {
       return cb(null, file);
     }