Implements Feature Request from #272

Headers are now stored as an array and operations have been changed to work with it. Rendering the header has been deferred to row creation, which is not ideal and should be adjusted.

Multiple table headers are now allowed, but they cannot be mixed with table rows.

May be exhibiting buggy behavior per #292.

Previously, header rendering was deferred until row creation. This is changed so tables can now be exclusively headers. Border behavior is also adjusted for header-only tables to ensure that bottom borders will be created for the last header row.

Test related to issue #292. The test isn't good as a basic test, but is worth keeping around.

Headers should render and have a bottom border if borders are included.

Table header rows would skip an index when a table with exclusively headers is rendered.

A simple test to showcase the use of multiple headers and how it looks spread out across multiple pages.