Allow finding issues on another repo & drop env. variables

Breaking changes:

• all outputs are following a common naming conventions:
  kebab case
  • issue_number is now issue-number
  • has_found is now has-found
  • is_closed is now is-closed
• environment variable were moved to action inputs
  • GITHUB_TOKEN is now token and defaults to github.token

Features:

• New optional input added: repository. Now you can fetch issues on other
  repository instead of on the one that the action runs.

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PR: #6