Add isArchived method handling archived property of Repository

[vchernodon]

The archived property included and exposed via isArchived method.
This helps avoiding to make any github calls which would result in error when the repository is archived.
So before making such calls (create comment/status etc.) - the isArchived method could be checked to avoid making other calls.

[codecov]

Codecov Report

Merging #85 (c029a8b) into master (b1b4cf2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #85   +/-   ##
=========================================
  Coverage     70.20%   70.20%           
  Complexity      222      222           
=========================================
  Files            36       36           
  Lines           866      866           
  Branches         36       36           
=========================================
  Hits            608      608           
  Misses          233      233           
  Partials         25       25           

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