added --ignore-pullrequests option

Adding option for filtering pull requests from list of issues.

As Github API does not provide a way to filter the request for non-pull request issues, it had to be done filtering the response

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https://developer.github.com/v3/issues/#list-issues-for-a-repository
> Note: Every pull request is an issue, but not every issue is a pull request. If the issue is not a pull request,   the response omits the `pull_request` attribute.