Add option to add a legend header or footer

[douggreen]

Description

This adds a very simple option to add a header or footer to the legend. I use this to add the printed date.

Form changes

(note that the image above also includes the "compact" option suggested in https://github.com/code4recovery/pdf/pull/45)

Output changes

[douggreen]

Closing this. We are now going to fork this repo to meet our immediate needs and work over a period of time to get those changes upstream.