Added pr_attached attribute to issue

- If it is true, PR is attached with the issue
- If it is false, PR is not attached with the issue
Migration for adding an boolean attribute skip_issues_with_pr to users
User can skip issues attached with pull requests by setting skip_issues_with_pr to true
Prefer refute over assert !
Merge pull request #166 from prathamesh-sonpatki/non-pr-issues

User can skip issues which already have pull requests attached to it