Feat verify failed rules

[logaretm]

This PR adds failedRules to the Validator.verify result and to the ValidationProvider slot-scope props, which is very useful if someone wants to do UI customizations based on the failed rules.

<ValidationProvider rules="required|email" name="field">
  <div slot-scope="{ errors, failedRules }">
    <input type="text" v-model="value">
    <span v-if="failedRules.required">Come on, this is required!</span>
    <span v-if="failedRules.email">Have you ever seen an email like the one you just wrote?</span>
  </div>
</ValidationProvider>

closes #1898

[codecov]

Codecov Report

Merging #1916 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1916      +/-   ##
==========================================
+ Coverage   96.11%   96.12%   +0.01%     
==========================================
  Files          60       60              
  Lines        2291     2297       +6     
  Branches      714      714              
==========================================
+ Hits         2202     2208       +6     
  Misses         80       80              
  Partials        9        9

Impacted Files	Coverage Δ
src/components/provider.js	90.9% <100%> (+0.04%)	⬆️
src/core/validator.js	98.57% <100%> (+0.02%)	⬆️

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