Add Greek translation

[waseigo]

Fixes the lack of Greek translation

Changes proposed in this pull request:

• Add el_GR directory in src/humanize/locale
• Add Greek-translated humanize.po file
• Add Greek language mention in the README

[codecov-commenter]

Codecov Report

Merging #46 (01bca3d) into main (bbc18e8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #46   +/-   ##
=======================================
  Coverage   99.13%   99.13%           
=======================================
  Files           9        9           
  Lines         690      690           
=======================================
  Hits          684      684           
  Misses          6        6           

Flag	Coverage Δ
macos-latest	97.39% <ø> (ø)
ubuntu-latest	97.39% <ø> (ø)
windows-latest	97.10% <ø> (ø)

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[hugovk]

Thank you!