#230 implement Info.makeLiveImg

[Angel Herraez]

The webpage templates of Jmol Export to Web module use a "makeLiveImg" key of the Info variable.
Apparently, that is not actually implemented in JSmol, but introduced with some trick.

Several users asked for this functionality (ie the ability to specify the path anf filename of a custom image file that is used overlayed on the cover image, bottom-left corner, to indicate "click to uncover", ie hide the cover image and show the JSmol behind it.
I think it would be useful and so I propose to implement it as standard in JSmol distribution

Either keep this name, "makeLiveImg", or use more meaningful one eg "uncoverImage" --we already have coverImage and this somehow does the opposite

[Angel Herraez]

Implemented in JSmol, JmolVersion="14.4.1_2016.01.15"
as Info.makeLiveImage