Make filtering of include explicit

This adds the syntax:

```jade
include:filter-name path/to/file.ext
```

It also means that if you had:

```jade
include foo.js
```

before and expect it to carry on adding `<script>` tags, you will need to switch to either:

```jade
include:js foo.js
```

or

```jade
script
  include foo.js
```