Add line spacing

This is similar to the `line-height` property in CSS. It is especially useful when trying to convert an HTML/CSS presentation into an image.

Note: Though line spacing is similar to CSS `line-height` it is important to know that there are differences. CSS `line-height` is calculated as the distance from one baseline to the next baseline. Line spacing is measured as the distance from the baseline to the top of the glyph (read: character) bounding box.