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

* 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

Implementation of https://github.com/mpetroff/pannellum/issues/151.

Now the following can also be set from URL:
  - minHfov, maxHfov
  - minPitch, maxPitch
  - minYaw, maxYaw

The motivation for this was making it easier to create auto-generated
panoramas using Jekyll/Hugo and other static site generators without
needing to create JSON files for panoramas.

If a given panorama is not a full 360x180 panorama, then the above
parameters are needed in addition to haov and vaov parameters
in order for the user not to see black areas in the panorama.
Hence suggesting adding URL support.