added design functionality

- added option to set an individual color for each section
- the individual color for each section, once set, overrides the
general header color set in “setHeaderColor”. if you want a header to
have the color set in “setHeaderColor”, just don’t give it an
individual color.
- added option to set individual image for each header.
- added option to set that one section will always be open.
- updated example app to showcase new features.

thanks to @orenk86