Added the ability to add content to multiple collections

This lets you do

```
---
title: My Awesome Page
collection:
  - Favourites
  - Travel
  - Shopping
---
```

or

```
---
title: My Awesome Page
collection: Favourites
---
```

This only thing I'm not sure of is what line 41 was being used for, since that will reset the collections array on the file.