Made symlinks maintain relative folder structure

For example,

```js
gulp.src('lib/**/images/*')
.pipe('symlink(./build'))
```

With project structure

```
lib/
  pages/
     cool-page/
         images/a.png
lib/
  components/
     cool-component/
         images/b.png
```

Would result in:

```
build/
   cool-page/
       images/a.png
   cool-component/
       images/b.png
```

This is more consistent with `gulp.dest`'s behavior.