Support central configuration file

There's now a central configuration file for your slides. In the future, it'll probably hold all sorts of stuff, but for now, I'm suppporting three things: author, title, and theme.

Here's an example of a config:
```
title = "The Title of your Slides"
author = "Your Name"
[presentation]
theme = "night"
```

It's in TOML. You can pick any theme that's supported by reveal.js: night, default, etc

This will fix issues #5 and #6.