Add validation of organisasjonsnummer, with tests

[perchrh]

Hi,
if you want it, you can add support for organisasjonsnummer to your module.
Perhaps the name of the module should change, as well, then, to "Offentlig idnummer" / "Norwegian National ID numbers" or similar.
I needed this code myself, in order to validate both orgnr and fnr, and share the additions with you.

[codecov-io]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #3   +/-   ##
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  Coverage     100%   100%           
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  Files           1      1           
  Lines          94     94           
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  Hits           94     94

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[magnuswatn]

Hi!

This looks great! I will take a closer look and merge later :-) Thanks!