Add currentPathIs, currentRouteIs helpers


I often find that I want to reuse a particular template in many places, but I may want one small thing to be different depending on the current route. A really neat thing in handlebars is being able to call other helpers from an {{#if}} or {{#unless}} block, so I wrote some generic helpers. I thought other people might find them useful as well.

You can use these helpers like this:

```html
{{#if currentPathIs '/posts'}}
    You're on posts!
{{/if}}

{{#unless currentRouteIs 'postsRoute'}}
    You're not on posts!
{{/unless}}
```