install scripts upgrades: display version installed, reinstall, upgrade, remove

[nixjdm]

Currently there is no provided way to uninstall Lektor if you installed via the install.sh / install.ps1.

The scripts should detect an existing Lektor version, if found display that version, and then prompt the user for reinstallation, upgrade, or removal.

This is motivated by lektor/lektor#556, which requested the ability to uninstall via the install script.

[blaisep]

@nixjdm Is there a reason we don't use setup.py ?
Then we could easily build options like --test , --uninstall , --upgrade etc. ? If it's just a matter of getting help, I could take a stab at it and submit a PR ...

[nixjdm]

We do have and use a setup.py, but that's more for developers, which not all of Lektor's users are. The shell script is meant to allow for an easy installation for people who are not Python devs. These scripts use pip behind the scenes.