Uppercase table headers

@charlierudolph I really like uppercased table headers. There are several reasons:
* table headers should be highlighted. That's why HTML has the `th` element. Ideal would be bold, but since we don't have that, uppercase is the next best option. 
* Seeing this implemented makes parsing a table so much easier. Just a quick glance at a table tells me exactly what it defines. Before I had to find the first column, and read through it with a lot more effort to get an idea.

I think this makes tables a lot more useful. What do you think?
I dislike this. Can you please move it out into a steps so it can be done with `map_headers!`?