Allow to configure the value set when the input is cleared out

[tochoromero]

Provide a property to specify the value it will be set when the input is empty.
Fixes #47

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   97.72%   97.77%   +0.05%     
==========================================
  Files           1        1              
  Lines          44       45       +1     
  Branches       16       17       +1     
==========================================
+ Hits           43       44       +1     
  Partials        1        1

Impacted Files	Coverage Δ
src/vue-numeric.vue	97.77% <100%> (+0.05%)	⬆️

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