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: 
Pushed up a fix for this so we only allow unnamed slides when running in the config file's context itself.

Good point about other config options, but I think we can easily do something like this where we change state on the config object--maybe by including a module after we've loaded the slides directory--so the options are only there when we want them. Will worry about that later, though, when we have other options (which I am considering adding, but haven't done yet...)