Add ENABLE_MAGICK CMake option (default ON)

[ferdnyc]

This will allow building with ImageMagick support to be switched off (using -DENABLE_MAGICK=0 on the cmake command line), even if it's detected on the system (which was previously not possible).

Also: Change the default build type (when not specified at all) from Debug to Release, which is more typically what's expected.

[codecov]

Codecov Report

Merging #574 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #574   +/-   ##
========================================
  Coverage    49.39%   49.39%           
========================================
  Files          129      129           
  Lines        10198    10198           
========================================
  Hits          5037     5037           
  Misses        5161     5161           

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[ferdnyc]

I'm going to merge this immediately since it's fairly trivial, and I'm hoping it might help solve some issues seen in #573 .