Use to_yaml instead of handbuilt serializer

This has similar functionality to #11 (and addresses Issue #5), but uses `to_yaml` as its yaml generator resulting in somewhat more verbose yaml. e.g.

Using `to_yaml`:
```yaml
---
title: Pool Soup
date: '2005-11-13'
tags:
- California
- filth
- leaves
- pool
- video
---
```

Using previous handbuilt serializer:

Before:
```yaml
---
title: Pool Soup
date: 2005-11-13
tags: California, filth, leaves, pool, video
---
```