v0.12: Hardened downloader

Biggest update in v0.12 is simplified, but also hardened downloader for docsets. I wanted to implement goroutines to speed up the process and download docsets in a lot of threads, but it’s fast enough in all my tests. So I kept it simple, but tried to make it as reliable as possible.

https://poshtui.com website now has also improved navigation and documentation page. It’s very simplistic, but it’s a start.

posh docsets --clear command now also compeltely removes all files from ~/.local/share/posh/ directory. This might get useful in future if sqlite database schedma will change.

I was also approved for paddle.com account and most likely my next update will be about licensing and introducing limits for freemium version of the app.