First pass at an `--editions` command

Also added a “description” field for wiki folders.

Right now there’s too many editions listed. I think we should by
default only list editions whose `tiddlywiki.info` file has
`showInListings` set to `true`, and have an `--editions all` command
that lists everything.

@pmario please could you check that the editions in your
`TIDDLYWIKI_EDITION_PATH` are correctly listed?