Installation script upgrade - add support for installing KonText with CNC custom Manatee-open from a repository

[tomachalek]

It should work like this:
In case the installation is run without --ucnk, then there should be no change
In case --ucnk is applied, the script should look first to a respective GitHub repository whether there is a matching version

• if true then it should install it
• else it should perform the old-way installation (i.e. download from "muni.cz" and apply the patch)