Fix jade includes

Using the 'include' feature of jade results in this error:

```
  1) Engines JST JADE should compile to a JS function:
     Error: [JadeEngine] Jade:3
    1| div
    2|   | template
  > 3|   include ./included
    4| 

the "filename" option is required to use "include" with "relative" paths  (in /Users/cgc/src/mincer/test/fixtures/jst_engine/jade/template.jst.jade)
...
```

This PR fixes that.