Assign class selector for modals
I'm using v2.0.2 and the selector for modal is for `a` tags. I'm using [router.js](https://github.com/ramiel/router.js) for a single page application and it uses links to switch views. The problem is this error is always thrown

```
Uncaught SyntaxError: Failed to execute 'querySelector' on 'Document': '#issues/0' is not a valid selector. ratchet.js:37getModal ratchet.js:37(anonymous function)
```

It does not change the router.js behavior but I think this is a good thing to have..

Please let me know if you have ideas for the selector name and if this should be done, I'm planning to submit a PR if needed.
