Add %% (escape) token to greeter

You can use %% to create a % that will not be replaced by other tokens. I used a small hack for this, a 16 bit hexadecimal random value that is generated at startup which serves as a placeholder that won't be replaced by other tokens. After all of them have been applied, the placeholder is converted back to the actual %. I had to use a list instead of a dict for this, because the dict does not provide a predictable order of the elements. (I used it as if it was a list anyways :smile:)