Add client side variables validation

[arbulu89]

Implement client side variable validation using the pattern option:

https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/pattern

It adds 2 new options to configure the variables using the description entry;

[title:my variable title [pattern:[mypattern]:endpattern]

I have not found a better way to encapsulate the pattern option without the endpattern keyword, as the patterns might have several [] (even nested ones).

[codecov[bot]]

Codecov Report

Merging #161 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #161   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          700       700           
=========================================
  Hits           700       700           

Impacted Files	Coverage Δ
app/models/variable.rb	100.00% <100.00%> (ø)
app/controllers/variables_controller.rb	100.00% <0.00%> (ø)

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[arbulu89]

Hi @bear454 ,

I have updated the keywords to something like [pattern:/my regex/]. It looks better and it still works fine