Non-destructivly merge multiple calls to meta

This patch allows multiple calls to meta without obliterating the previous settings.

Personally I find

```ruby
meta page_size: [612, 1008]
meta margin: 0
meta background: "/path/to/bg.jpg"
```
easier to read than

```ruby
meta page_size: [612, 1008], margin: 0, background: "/path/to/bg.jpg"
```

This PR doesn't include any updates to the tests because I'm not entirely sure the best say to test this, short of using dependency injection to place a mock inside `PrawnCocktail::Renderer#prawn_document` and check to make sure the correct options where passed in. That's a bit more work than I was willing to put in without checking to make sure that would be accepted before hand.

Thoughts?