Prevent hiding of partial lines of output or previous command by the prompt

[anokun7]

When the output of the previous command does not end with a newline, the prompt hides that output. You can however see it for a fraction of a second. This PR fixes that behavior. By default a ‘%’ is appended to the partial line output for a normal user (and a ‘#’ for root). This can be customized by setting the value of the env variable PROMPT_EOL_MARK