Integer and Float Cols

Thanks for this package; it is great and HTML is fiddly and all of my tables are basically the same.

Subclassing Col is easy enough, but it would be nice if there were support for numeric Cols out of the box. The main ones I was thinking of were:

• Integer
• Float
• Percent

Alternatively, since the td_format in each of these cases is going to be pretty lightweight, allowing a formatting function to be passed as an argument to Col would be pretty nice and flexible.