More tests. More coffee. More attributes for .md slides.

Hi, here are the promised unit tests :) and a helpful new option.

* Add unit tests for all existing options.
* Move implementation to coffee script. *Oh my god, this is soooo readable* :)
* Add option `--attributes` for markdown slides.
* Clean up a few things (e.g. naming `_.templates` with a leading `_`, declaring options in `SlideGenerator`)

With this, running `yo reveal:slide "md-with-background" --markdown --attributes` yields 
```json
[
    "index.md",
    {
        "filename": "md-with-background.md",
        "attr": {
            "data-background": "#ff0000"
        }
    }
]
```
which in turn yields a *red background* on that slide. I find this very useful, when working with `.md` slides. Of course, this is not limited to setting the `data-background` attribute.

(@knuton, these may be the drones you are looking for:))