Reworked options() from 0.9.0 version

[bigcat88]

As we come closer to 1.0.0 version we need more consistent API for setting options.
Was looking how it is done in Pillow and liked it.

Current code:

pillow_heif.options().thumbnails = False

in new version will be changed to:

pillow_heif.options.THUMBNAILS= False

Usage when it is used a plugin will not be changed, old code will work the same way:

from pillow_heif import register_heif_opener
register_heif_opener(thumbnails=False)

It will require less code from me to support, will not break API for plugin use, and such code will be a bit faster.

Changes for code when use it not as a plugin:
pillow_heif.options().thumbnails=False -> pillow_heif.options.THUMBNAILS=False
pillow_heif.options().quality=100 -> pillow_heif.options.QUALITY=100
pillow_heif.options().save_to_12bit=True -> pillow_heif.options.SAVE_HDR_TO_12BIT=True

New IMPORTANT options (list will be updated as I currently working on this):

DECODE_THREADS=4
Option to set number of decode threads(when it is possible). Benchmarks will be posted after update released.
Usage of option for register_heif_opener :

register_heif_opener(decode_threads=10)

or

pillow_heif.options.DECODE_THREADS= 10