Introduce method to remove defined sections from the builder

Follows the pattern laid out by add_section()
Correct documentation for add_section() and ttfmake_add_section
Merge branch 'remove-section' of https://github.com/jeremyfelt/make into jeremyfelt-remove-section
Check if section exists before removing.
Docs cleanup.
Merge branch 'jeremyfelt-remove-section' into develop