Fallback filter

The fallback filter replaces nil input with a default value. If both are hashes they will be merged.

Usage in #1814:

```
{{ page.modified | fallback: page.date | date_to_xmlschema }}
```

Another example for https://github.com/jekyll/jekyll/pull/969#issuecomment-20344309 from @benbalter:

```
{{ page.author | fallback: "John Doe" }}
```

I even thought the fallback filter gives the user a more straight forward tool than #1527. I don’t like the magic behind the default front-matter.