Add --prune flag to tarbell publish to delete files in s3 bucket

[bepetersn]

For instance, awhile ago I published to here, when a certain slide.html file existed in my project. But after later deleting the slide.html file from my project, and re-publishing, the old publish is still visible at:

http://chicago-justice.smartchicagoapps.org/slide.html#call

[eads]

Yeah, at the moment you have to manually prune old files. It's just a rather dangerous thing ...

[bepetersn]

Thought this might have been a #wontfix kind of thing.

[eads]

This is definitely a #willfix, it's also just #nicetohave because managing an s3 bucket is pretty easy via the web interface and clobbering remote files is potentially quite dicey. We need:

• A --prune flag for tarbell publish
• tarbell unpublish (see #25)
• A backup mechanism when these dangerous operations are carried out

[eads]

Moving out of 1.0 milestone, see #223