add option for multiple posts per date

[modgethanc]

currently, you can only make one post per date; it might be useful to be to opt for more than one post a day. this could tie in well with privacy/publishing options, to selectively make things public or not.

[modgethanc]

after sitting on this for almost a year (!), i think what i'd like is for a maximum of two posts per date, with one post at each privacy level. so, if you want to have a published post and an unpublished post on the same date, that's okay.

i'm leaning against being able to create even more than that; i never intended ttbp to be a microblogging/multiposting sort of thing, and i also kind of prefer the sort of rolling continuity of 'on this day you have this text file at your disposal; on a new day, you get a new text file'. that's just the sort of experience i want people to have here, i guess!