Add Base.Regex from RegexString with flags

[singularitti]

As mentioned here.

[codecov-io]

Codecov Report

Merging #6 (d3dbf4b) into master (3d681be) will decrease coverage by 1.28%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   84.61%   83.33%   -1.29%     
==========================================
  Files           1        1              
  Lines          65       66       +1     
==========================================
  Hits           55       55              
- Misses         10       11       +1     

Impacted Files	Coverage Δ
src/ReadableRegex.jl	83.33% <0.00%> (-1.29%)	⬇️

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