add linux support

[agolzer]

"base64 -D" is valid only on macOS - for linux a lowercase "-d" needs to be used. I added condition to use uppercase for macOS (Darwin) and for all other operating systems a lowercase "-d"

[georgettica]

hit the same issue and +1 this change..

it even is cleaner! @clkao can you merge?

[kozak-codes]

+1