Add support for metadata ContactPerson and Organization

[Its-Just-Nans]

Description

Hello,
What do you think about this ?

• Add the possibility to add more fields in the metadata
• Update package.json to support the type Buffer
• Add type
• Add test

Checklist:

• Link to SAML spec: https://docs.oasis-open.org/security/saml/v2.0/saml-metadata-2.0-os.pdf it's quite general :
  • Element <Organization> (page 12)
  • Element <ContactPerson> (page 12)
  • AdditionalMetadataLocation> (page 14)
• Tests included
• Documentation updated (in README.md)

[codecov]

Codecov Report

Merging #140 (438724b) into master (718508e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   81.06%   81.10%   +0.04%     
==========================================
  Files          13       13              
  Lines         829      831       +2     
  Branches      242      244       +2     
==========================================
+ Hits          672      674       +2     
  Misses         71       71              
  Partials       86       86              

Impacted Files	Coverage Δ
src/types.ts	100.00% <ø> (ø)
src/metadata.ts	90.47% <100.00%> (+0.47%)	⬆️

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