enable viewing locally defined panoramas like the examples

[DDvO]

Fixes #572 (and likely also #175)

[mpetroff]

I think this is likely to cause more confusion, since it only works in certain cases. The current error message at least makes it clear why it doesn't work. I also don't want to move pannellum.htm to the root directory.

I'm going to have to think about this more.

[mpetroff]

Have you tested this in Chrome? I can only get it to work in Firefox.

[DDvO]

As I just wrote on #572, this works with Chrome (and thus also with Chromium) after a further tweak of standalone.js and pannellum.js when it is invoked with the --allow-file-access-from-files command-line option.

[kanalasumant]

Awesome, you guys seem to be on top of this. How about opening an issue with the chromium, so that in the future they may incorporate this feature without any feature flags. If they do not wanna proceed with this proposal, that's their call, but asking when needed seems to be the next logical thing here. @mpetroff @DDvO what do you think about this.

[DDvO]

At least at the moment I do not have the time to discuss with Chrome/Chromium developers the underlying security implications, which appear rather intricate.

I've just improved the workaround for Pannellum proposed in this pull request.
Now the error message is given only if local file access actually has been denied.

I tested also with other browsers: Safari acts like Firefox (i.e., it does not care),
while Opera behaves like Chrome and provides the same command-line switch.