Add feature to exclude pages again

[mbaeuerle]

Currently in Briss 2.0 there is no way to exclude pages like it was possible in the classic Briss 0.9.
As a first step it might be enough to just have an input field in the main Gui where excluded pages can be set.

[cleydyr]

What does it mean exactly for a page to be excluded? Is it that the final PDF won't contain the page or that it can't be cropped?

[GitHubRulesOK]

@cleydyr
Hmm...
"skip" was an odd nom de plume as using say "skip 1" v0.9 cropped the cover independent of other pages (so more like split crops about 1)

[cleydyr]

Closing as #58 has been merged.