Include the file's directory in includePaths

When trying to render an scss file, which requires a file in the same directory, it will fail, since sass cannot find the file. Including the root file's directory in `includePaths` fixes this.

Before the following would fail:

```
// main.scss
@import "test";
```

It would yield the following error:

`Error: While reading or transforming "path/to/main.scss": source string:2: error: file to import not found or unreadable: "test"`

The usage of `includePaths` fixes this. Local file imports without manually passing includePaths, yay.