Template header & template footer added to options

I use gulp-angular-templatecache with gulp-ng-autobootstrap & webpack. 

It would be nice, if templates can be changeable according to the developer's needs.

As you can see below, i need to change templates to
```javascript
var s = gulp.src('./templates/**/*.html')
    .pipe(ngTemplateCache({
        moduleSystem: 'browserify',
        templateHeader: '["$templateCache", function ($templateCache) {\n',
        templateFooter: '\n}];'
    }))
    .pipe(gulp.dest('./public'));
```

Output:
```javascript
module.exports = ["$templateCache", function ($templateCache) {
	$templateCache.put("template1.html","<p>template 1</p>");
	$templateCache.put("template2.html","<p>template 2</p>");
	$templateCache.put("template3.html","<p>template 3</p>");
}];
```