Add `options=None` to Posts.get_....() methods.

[atamaokac]

This PR adds optional argument options=None to:

• Posts.get_list_of_flagged_posts()
• Posts.get_posts_for_channel()
• Posts.get_unread_posts_for_channel()

In some cases I found it necessary, e.g., when we want to get only metadata of very many messages.