[Feature Request] Format merging (aka multi-format issue entries)

[jpcranford]

When YACReader imports/updates new files, it displays a separate comic for each file. It would be nice if we could merge two formats (say, PDF and CBZ versions of the same issue) so the Library would display one issue, with metadata attached to it, then give us the choice of which format to open, similar to how Plex and Calibre handle this.