Adds .cant_split and .cant_split? table methods.

[benjaminketron]

.cant_split allows one to describe a row or rows in such a way
that its contents cannot split across a page break and instead
positions the entire row on a new page.

For example

docx.table [['Header 1', 'Header 2'], ['Cell 1', 'Cell 2']] do
  cant_split rows[1]
end

or

docx.table [['Header 1', 'Header 2'], ['Cell 1', 'Cell 2']] do
  cant_split rows[1...-1]
end

After reviewing this with my colleagues the consensus is we might want to start gathering rows in a new TableRowModel where this type of thing can be kept. This PR might be updated relatively soon.