Added recursive partials parsing and compiling

Added new optional option ```parsePartials: true``` in this case will be found all partials inside the templates and registered/wrapped into appropriate instance/module.

```js
var handlebars = require('gulp-handlebars');
var defineModule = require('gulp-define-module');

gulp.task('templates', function(){
  gulp.src(['templates/*.hbs'])
    .pipe(handlebars({
        parsePartials: true
    }))
    .pipe(defineModule('node'))
    .pipe(gulp.dest('build/templates/'));
});
```