rework dispatches and update compatibility

[johnnychen94]

The purpose of this rewrite is to close #22, close #28 and close #25 . Also, it's much easier to understand and maintain the source codes.

This rework consists of three updates:

• dispatch result_type on ::Type rather than ::AbstractArray ( ~blocked by https://github.com/JuliaStats/Distances.jl/pull/140~)
• dispatch eval_op instead of evaluate. The latter introduces a lot of method ambiguities (e.g., https://github.com/JuliaImages/ImageDistances.jl/issues/28#issuecomment-495010897 reported by @tlnagy)
• Compatibility update: require ImageCore >= 0.8.3 --> I'd like to make this a seperate PR but it's required to simplify the dispatching.

Todo:

• [x] Color3 support
• [x] Hausdorff and CIEDE2000

cc: @juliohm

[johnnychen94]

Unfortunately, ambiguities still exist (moves from evaluate to result_type). I plan to fix it in the next PR.

[codecov[bot]]

Codecov Report

Merging #32 into master will increase coverage by 18.5%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage    69.1%   87.61%   +18.5%     
==========================================
  Files           6        6              
  Lines         123      113      -10     
==========================================
+ Hits           85       99      +14     
+ Misses         38       14      -24

Impacted Files	Coverage Δ
src/ImageDistances.jl	100% <ø> (+25%)	:arrow_up:
src/metrics_distances.jl	100% <100%> (+37.5%)	:arrow_up:
src/hausdorff.jl	86.95% <33.33%> (+49.74%)	:arrow_up:
src/metrics.jl	66.66% <50%> (-22.23%)	:arrow_down:
src/ciede2000.jl	87.5% <83.33%> (-2.5%)	:arrow_down:
src/generic.jl	94.87% <84.61%> (+4.87%)	:arrow_up:

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