feat: add ability to select multiple packages

[darukutsu]

fixes #26

• for now there is problem after installing/uninstalling not clearing visual marks due to we list cached search... will fix later
• needs testing
• also can you provide me cli examples for all the TODOs marks in code, so I can implement them somehow, i have no idea what it looks like, maybe document those also in manpage as well
• code itself is trash, I don't like how it looks feel free to change/optimize it
  edit: I'm more or so satisfied...
• add ability to mark for reinstallation

Quick video how it works for now:

pacseek.mp4

[darukutsu]

now we cooking :D... Ok so fixed my bad code now marks will correctly appear disappear in multiple pages cuz we're flushing cache. I made it so even if we abort halfway through installation, apps which were marked, but not installed will have preserved marks, so we can modify install list.

I still need examples for those TODOs in commands.go I mean {giturl},{optdepends}... so I can finally implement those and finalize this PR.

Hmm... my video of how it works wasn't uploaded and for some reason I can't upload any type of file nor mp4, gif, webm. Well ¯_(ツ)_/¯ for anyone who wants to try <Space> selects/deselects and pressing <Enter> on item while having in selection others, will install that item too.

[darukutsu]

I don't know go that well, I need to access this variable in 2 different files. I think defining it on top of setup.go should work too, but not sure if proper place for it. Also I'm passing it as copy slice not as pointer, then functions return new modified slice ,not sure if it's deal-braker.

[filip-rs]

Worked fine on my machine. Nice use of spacebar that's otherwise unused. Intuitive and I'd be in favour of merge.