Feature/removes validatable field typealias

[YANOUSHek]

Hi,

I've recently had the pleasure to use SwiftValidator in my own project and I've run into a couple issues, so here are my fixes for them:

1. Makes Validator an open class to it can be inherited from and modified for your own needs.
2. Removes the weird fields property from Validator which made completely no sense.
3. Removes the ValidatableField typealias by making Validatable protocol available only to classes.

[codecov-io]

Codecov Report

Merging #176 into master will decrease coverage by -0.04%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
- Coverage   80.82%   80.78%   -0.04%     
==========================================
  Files          26       26              
  Lines         485      484       -1     
==========================================
- Hits          392      391       -1     
  Misses         93       93

Impacted Files	Coverage Δ
SwiftValidator/Core/Validatable.swift	50% <ø> (ø)	✅
SwiftValidator/Core/ValidationError.swift	100% <100%> (ø)	✅
SwiftValidator/Core/ValidatorDictionary.swift	62.5% <100%> (ø)	✅
SwiftValidator/Rules/ConfirmRule.swift	100% <100%> (ø)	✅
SwiftValidator/Rules/ValidationRule.swift	100% <100%> (ø)	✅
SwiftValidator/Core/Validator.swift	73.07% <80%> (-0.51%)	❌

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2a6c23a...a021c44. Read the comment docs.

[jpotts18]

Sorry for the delay @YANOUSHek I don't do Swift in my day-to-day and this project is suffering. Would you be interested in becoming a contributor?