Issues with prawn's automatic column_width decision making

[whysthatso]

render_line_items is giving me headaches, because it cannot be coerced into rendering different use cases (which i cannot even replicate sensibly) of amounts of columns, and content width or title width.

so i tried to figure out how to extend the parameters of render_line_items and came up with a PR for that feature. it's optional, so if not needed, users can ignore it.