Simpler way to define slides

Addresses #6, just for fun.

Now you can define things right in `config.rb` like this:

```
slide do
  centered_huge_text 'Welcome to Wingtips!'
end
```

You can also use the dsl in `slides/*.rb` files, although you'll have to give them a name to reference them later via `slides` in config, like so:

```
# slides/title.rb
slide "Title" do
  centered_huge_text 'Easy content, hurray!'
end

#config.rb
slides Title
```

Zing! :rocket: