feat(#145): rework hint to status message that can be updated when status is changed

[MatousJobanek]

• reworked hint to a more elaborated status message
• status message can be updated
  • this is used in test-keeper where different messages are set for the cases when either the PR is updated by commit containing a test or is updated so it contains only skipped files - for more information see this comment https://github.com/arquillian/ike-prow-plugins/issues/145#issuecomment-397629389
  • for the happy (success) comments, there is used a happy Ike's image
• the size of Ike's image is smaller
• there is only one possibility of customizing the messages - using .md files in .ike-prow/ directory
  • the files that are checked are:
  • test-keeper_without_tests_message.md for the case when no tests is added
  • test-keeper_with_tests_message.md for the case when PR is updated by a commit containing a test
  • test-keeper_only_skipped_message.md for the case when PR is updated so it contains only those files that are skipped

Fixes: #145

TODO:

• [x] update documentation