Improve readability by separating the table structure from table-def

This adds a <table> abstraction that combines each of the elements of a
valid table; in turn, this allows <table-def> to be more clearly
written.
Improve BNF references to manpages and use comment symbol '#'

Alignment is improved for better visual reference, and the asterisk in
the definitions is removed. Also referenced the <module> definition.
README: add to operational modes; define line types & table defs