add the css classes _0 _1 _2 _3 ... to table headers

[bjelline]

add the css classes _0 _1 _2 _3 not just to td tags but also to the header th tags,
making it possible to style all the columns.

[gordonwoodhull]

Thanks @bjelline, this is more consistent and a good idea. It is potentially a breaking change for anyone who wrote selectors just on _1 without specifying td, so I'll schedule it for 2.1.