Pass options to swig.setDefaults

swig cache is enabled by default.  If you use this plugin with gulp.watch, it won't pick up on HTML changes.

with this change you can pass `{cache:false}` and other options thru to swig...

```js
gulp.src('./src/*.html')
    .pipe(swig({load_json: true, cache: false}))
    .pipe(gulp.dest('./dist/'))
```