Improvements for comic book conversion for Pocketbook Basic Touch 624

I was trying to  convert some comic books to PDFs for my PocketBook Basic Touch 624 and I had some trouble, so here are patches that resolved my issues:

1. the `--wide` option preserves aspect by default, which I didn't like, because it added white borders on the sides, which kinda defeats the purpose of the `--wide` option to maximize visible area in landscape mode. Therefore, I made aspect preservation optional an it is already controlled by the `--keep_aspect_ratio` switch (yes, I did reflect it in the documentation)
2. none of the available output profiles fully matched my device, so I added it

As a side note, `--wide` adds 25 pixels for some battery bar, however, such behavior suits just some devices and should be optional. I understand that if a profile does not specify a `comic_screen_size`, the 25 px are subtracted from the `screen_size`. This should be clearly documented, otherwise people will get oversize output with e.g. the `PocketBook900Output` profile.

Keep up the great work :)