The filename option needs to be set to use the extends keyword

Jade was throwing an error because the filename option wasn't set when trying to use the extends keyword (for template inheritance)

```
stream.js:81
      throw er; // Unhandled stream error in pipe.
            ^
Error: the "filename" option is required to use "extends" with "relative" paths
    at Object.Parser.resolvePath (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/jade/lib/parser.js:399:13)
    at Object.Parser.parseExtends (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/jade/lib/parser.js:418:21)
    at Object.Parser.parseExpr (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/jade/lib/parser.js:210:21)
    at Object.Parser.parse (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/jade/lib/parser.js:133:25)
    at parse (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/jade/lib/jade.js:93:62)
    at exports.compile (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/jade/lib/jade.js:154:9)
    at jade (/home/vagrant/frontend/node_modules/gulp-jade/index.js:13:20)
    at Stream.module.exports.stream.write (/home/vagrant/frontend/node_modules/gulp-jade/node_modules/event-stream/node_modules/map-stream/index.js:57:28)
    at Stream.ondata (stream.js:38:26)
    at Stream.EventEmitter.emit (events.js:96:17)
    at next (/home/vagrant/gulp/node_modules/event-stream/node_modules/map-stream/index.js:45:25)
    at module.exports (/home/vagrant/gulp/lib/readFile.js:7:5)
    at fs.readFile (fs.js:176:14)
    at Object.oncomplete (fs.js:297:15)
```