Digitsep: Separate digit groups in integers

Convert a number to a string, separating each 'k' digits by 'sep'.  'k'
defaults to 3, separating by thousands.  The default "," for 'sep'
matches the commonly used digit separator in the US.
Merge pull request #1 from julian-gehring/digitsep

Digitsep: Separate digit groups in integers
Update README.md
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