Improve implementation of staggered columns.
 
Perhaps move interpretation of .lf commands with tables into
the table_input class.

Think about multi-page tables without .TS H/.TH. At the beginning of a
section we could transparently output a need request that references a
number register that we could set at the end of the section. This
won't work for boxed tables, because there's no way to draw the top
rule, and it won't work for tables with vertical rules unless the
macro package calls .T# even for tables that don't start with .TS H.

Support fractional point-sizes: both `linesize' option and `p'
modifier.

Make it possible to split a table section that's longer than a page.