Added check for common package managers instead of assuming apt-get

[Nosen92]

The linux install script assumed that the package manager always was apt-get. Instead of hardcoding for apt-get, the script now checks for common package managers and uses the one it can find.

The compatible package managers are:

• apt-get
• dnf
• pacman

which should cover almost all linux machines. More can easily be added in the future.

Minor tweak: Changed (y/n) to (y/N) to highlight default.

[DRGN-DRC]

Looks like a good change. I assume you've tested the script when using these other managers?

Before merging, do you want to add yourself to the credits at the bottom of the ReadMe and MD5s.txt file?

[Nosen92]

Sure, I'll do that