Support for non-dismissable modals

Hi there!

A couple days ago, I realized that I needed a modal to prompt users for acceptance on the privacy policy of my webpage, for example, to allow installation of third-party cookies.

The problem is that many users simply dismiss these kind of modals, by pressing `esc` or clicking outside the modal (in the `#lean-overlay`).

In order to disable this behaviour, a simple option can be passed to the modal, by doing:

```javascript
$('.modal-trigger').leanModal({
  dismissable: false
});
```

... or if it was to be opened programmatically:

```javascript
$('#cookie-policy-modal').openModal({
  dismissable: false
});
```

Along with a minimum modification on the `leanModal.js` file:

```javascript
if (options.dismissable) {
  $("#lean-overlay").click(function() {
    $(modal).closeModal(options);
  });
}
```

By default, the `dismissable` option is set to `true`, so no previous behaviour changes at all.

I thought this could be helpful for anyone out there! (this is actually my first *open* pull request... forgive me if the intended workflow is not this one!)

Kind regards!