Rework what arguments are passed to maskers

[skalee]

• For clarity and predictability, pass all attr_masker options to masker in a single hash.
• Pass the attribute name as well.
• Add specs for that, and for Attribute#mask method in general.

[codecov]

Codecov Report

Merging #92 into master will increase coverage by 37.66%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #92       +/-   ##
===========================================
+ Coverage   61.78%   99.45%   +37.66%     
===========================================
  Files          26       24        -2     
  Lines         560      366      -194     
===========================================
+ Hits          346      364       +18     
+ Misses        214        2      -212

Impacted Files	Coverage Δ
lib/attr_masker/attribute.rb	100% <100%> (ø)	⬆️
spec/unit/attribute_spec.rb	100% <100%> (ø)	⬆️
spec/spec_helper.rb
spec/features/shared_examples.rb
spec/features/mongoid_spec.rb	100% <0%> (+10.52%)	⬆️

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