Mark variables as const where appropriate

[Miepee]

Variables like cfgFile or cfgKeysFile should be marked as constants, as these are never expected to change.

[Miepee]

Partly addressed by #11 but not completely fixed yet.
There are still a bunch of variables that either a) should not be global or b) should be marked as constants.

[KarstSkarn]

Hello there! I did update the whole program today. I've been loosely working on it in my spare free time. I did include many things from your Huge Refactoring branch and fixed many of the "TO DO" you did write there. I also mentioned in the $Credit command inside the program (Just a thanks to you and your GitHub URL). If you have some spare time I invite you to take a look to it.

I didn't keep the files separated since the original source I kept working on it was the original old one. But I did manually upgrade some of your parts into there! @Miepee