add defaults method to presentation

- defaults get instance_eval'd along with other presentation methods
- defaults can set slide defaults only
- slides override defaults with merge of **kwargs

This is a quick solution that allows for the smallest amount of user configuration possible. From here, it's easy to add the defaults into the grouped slides. I haven't yet because I'm still testing the waters, learning the gem, and mostly I want to know what the author thinks about implementing configuration _this_ way. I'm open to suggestions/revisions/complete rewrites.