Allow gpgcCreateCommentIssue.sh to work with custom post_title

[selivan]

Different posts may have the same titles: for example, "Monthly report". So I would prefer to use page.id instead of page.title.

I can set post_title=page.id when including gpgc_comments.html, but gpgcCreateCommentIssue.sh is hard-coded to work with page.title.

It would be convenient to create option in gpgc.yml for this.