auto update

[Vincenzoferrara]

I'm on Linux, without Docker.

Auto download doesn't exist, so I have to manually install the latest version.
(Or at least, reading the code, I didn't see auto download.)

Maybe I could write a solution for auto download. In Go, or just for Linux in Bash.

[seriousm4x]

I've released a new version with auto update. You can now just use ./upsnap update.
f8a2654