Make templating engine configurable

I'm integrating RAML compilation into another documentation project where we use [Jade](http://jade-lang.com/) and I'd like to re-use my templates. This adds generic templating by using [consolidate.js](https://github.com/visionmedia/consolidate.js/). Per default handlebars is still used and hence remains the only templating dependency.

An example for usage with Jade would be:

```javascript
raml2html.parseWithConfig('api.raml', {
	template: 'template.jade',
	templateEngine: 'jade',
	templateOptions: { pretty: true },
	helpers: {
		md: function(markdown) {
			return markdown ? marked(markdown) : '';
		},
		lock: function(securedBy) {
			if (securedBy && securedBy.length) {
				var index = securedBy.indexOf(null);
				if (index !== -1) {
					securedBy.splice(index, 1);
				}
				if (securedBy.length) {
					return ' <span class="glyphicon glyphicon-lock" title="Authentication required"></span>';
				}
			}
		},
		highlight: function(code) {
			return code ? highlight.highlightAuto(code).value : '';
		}
	}
}, function(result) {
	console.log(result);
}, function(err) {
	console.error('ERROR: %s', err);
});
```

Along with the templates from [this Gist](https://gist.github.com/freezy/a291cec82d3c1aab784c).