Added options page

Container suffix: ZIP, CBZ
Page image format: PNG, JPG
and zip compression mode: NO COMPRESSION, DEFLATE

can now be selected by the user.
Improved presets and UX

- Default page type setting is now JPG. PNG has to be enabled explicitly
- Orange bar now shows the currently active settings to let the user
know what will be downloaded.
- After download the page that was opened before backup is now
automatically opened again.
UI improvements and page by page download

- Downloads can be made image by image now. Hopefully this will make it
possible to download larger comics. This currently may cause a crash
when trying to download PNGs
- Downloads can be started from any page now. If a file wise backup
failed, you can now restart where you left off.
Merge pull request #31 from Cortys/master

UI improvements and page by page download