Add a new filter: default

I found the filter default witch exist in the shopify document very helpful for me when i try to send a bulk emails.
It work perfect when you know your customers didn't have  same information, someone may have gender and the other may not. (or a complicated gender which they didn't want to let us know )

Example:

```nodejs
Dear {{ customer.name | default: "customer" }}
```

returns "Dear customer" if we don't know the name, but "Dear John" if we do.

http://docs.shopify.com/themes/liquid-basics/output#default