Allow custom colors with env variables

[eljulians]

As proposed in issue #85, I modified k to allow color customization with env variables.

Please feel free to suggest any change or to make them yourself. :)

[supercrabtree]

Hey, thanks for this. What operating system are you on? If you are using Mac then you can control the colours using $LSCOLORS ala https://unix.stackexchange.com/questions/2897/clicolor-and-ls-colors-in-bash#2904

If you are on linux you could use $LS_COLORS although as I am not on linux I have not implemented this yet.