1) If I wanted to write a plugin that works on a per-post basis, how would I do it? - Ideally, there should be a dropdown box in the admin New Post interface that lets me choose.
I want to be able to apply the plugin only to one particular post, or to a string of characters in the post, not to all posts in general.
2) If I turn on a plugin, say, MarkDown, will the articles published before the time I activated the plugin be affected as well? Likewise, if I turn it off, after publishing a post, will the MarkDown filter still work on that post?
Thank you in advance. :)