preferred-decoder option

[bigcat88]

Ref: #154

Changes proposed in this pull request:

• Implemented PREFERRED_DECODER in the same way, as PREFERRED_ENCODER in Preferred-encoder global option. #192
• PREFERRED_ENCODER now does not raise RuntimeError when specified encoder is not found.
• Added support of preferred_encoder and preferred_decoder keys to "register_avif_opener"/"register_heif_opener" function to easier set it in WSGI applications.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (17e7bb2) 100.00% compared to head (a26c28b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #193   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          912       923   +11     
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+ Hits           912       923   +11     

Files	Coverage Δ
pillow_heif/as_plugin.py	100.00% <100.00%> (ø)
pillow_heif/heif.py	100.00% <100.00%> (ø)
pillow_heif/options.py	100.00% <100.00%> (ø)