Hello,

the minified built version of pannellum.htm was not correct cause this space in the replace line was not replacing the corresponding line in src/pannellum.htm

        <link type="text/css" rel="Stylesheet" href="css/pannellum.css"/>

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.