Sometimes more than just the target attribute needs to be customized on the hotspot. Expansion of ba35ea8

Hotspots now accept an object with a list of attributes to append to the anchor tag.

Normally, the value of `minHfov` is ignored for said panoramas, and an
automatically calculated value is used instead.

Sometimes it is needed to open HotSpot link right in the same frame ("_self") or in a named one.

Change computation of cube size - taking haov into account

Removed wrong encodings of URLs (fixes #611).

Support parallel requests for multiRes panorama tiles

* support also partial cubemap/fallback images: backgroundColor for missing face(s)

* extend multires/generate.py to cylindrical input and partial panoramas

* fixed a few typos in new help strings of generate.py

* Missing cubemap faces now need to be defined as null in the configuration

* made generate.py strict again on existing output directory, printing descriptive error message

* replaced unsafe eval by ast.literal_eval in generate.py

* a couple of minor improvements regarding 'undefined' in libpannellum.js

* made generate.py allow existing output directory in debug mode

for convenience, no need to delete output directory every time while testing

* prevent display of out-of-range areas of partial (cylindrical) panoramas

by adaping min/may yaw and max horizontal fov, this no background (empty space) shown

* improvements as requested: made view restrictions optional, etc.

* extend documentation to describe new avoidShowingBackground option

This reverts commit 2491c2b4dc.

Avoid loading invisible files for partial panos of any image type.