Enhancement #857 - Implemented -p flag

[Alex-1611]

I have added -p (--slash-indicator, didn't know how to name it) flag from the windows ls command, which appends / to the end of directories name.

I have used the indicator flag's code to implement it and also added tests. I modified the render function to only work on directories and to not work while the display_indicators flag is also true. I also added the flag to the config file text, and set the default to false.

Let me know if there's anything wrong, or that should be changed.

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