Save Bool image with 1-bit depth

[timholy]

Fixes JuliaIO/FileIO.jl#232

[codecov]

Codecov Report

Merging #172 into master will increase coverage by 0.85%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   71.11%   71.96%   +0.85%     
==========================================
  Files           3        3              
  Lines         367      371       +4     
==========================================
+ Hits          261      267       +6     
+ Misses        106      104       -2

Impacted Files	Coverage Δ
src/libmagickwand.jl	67.82% <100%> (+1.15%)	⬆️
src/ImageMagick.jl	75.75% <66.66%> (+0.56%)	⬆️

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