Consider publishing this as a library crate as well as a binary

At least the types on `src/quadlet` could be very useful to be able to programmatically create quadlet files

```rust
use podlet::quadlet;

let mut file = quadlet::File {
name: "my-unit".to_owned(),
// ...
};

file.downgrade("5.6".parse().unwrap());

let contents = quadlet_file
.serialize_to_quadlet(&quadlet::JoinOption::all_set())
.unwrap();

// write contents to disk
```

I was sad to find this is not already the case as I could use it for some projects