Avoid ENOENT if target subfolder does not exists

I have a project with some less files located in a sub-folder of assets; something like:
```
assets
  myapp
    css
      main.less
      menu.less
```

When I try to compile this, I get a ENOENT exception because the `myapp` folder does not exists in the target folder (the less compiler tries to create the css folder, but the parent does not exists).

The current less compiler already handle that case but to do this, it depends on the nodejs mkdirp package.

Unfortunately, this has to be manually installed outside of sbt.

To avoid this, I suggest to create the parent folders in the scala code instead of in the javascript one.

I only modified the scala part; the javascript part can probably be simplified if this is applied.