Allow displaying images and video in hotspots

...but they don't really make sense content wise

Meaning that the css keyword in json is replaced by a header keyword
taking an html string as value. Example file updated accordingly

Useful for using, (for example) fontawesome icons for hotspots

Multires python script improvements

Replaces the os.system call for nona with a subprocess.check_call call
to get informed about errors when calling nona instead of silently
ignoring them before failing with missing files.

Use a formatter for argparse to print default values instead of doing
this manually.

Checks for a nona executable and adds a command line argument to specify
an executable. The argument is required from the user in case nona
cannot be found on the path.

Encode the explicit requirement on python 3 in the shebang line for
script usage.