Add ranger indicator

[MatejKastak]

Hello, I am using ranger which allows you to spawn a new terminal via shortcut S. The terminal will have $RANGER_LEVEL set, and it is advantageous to see if this shell is in ranger.

This PR implements: ranger prompt indicator

There is a possibility to show current RANGER_LEVEL in prompt setting theme_display_ranger_level to yes.

• How to handle theme and color settings?

[bobthecow]

Hello me understand what spawning a shell in Ranger means :)

[MatejKastak]

Hello, I am sure you have understood what spawning a shell in Ranger means. I just wanted to be helpful.

Regarding the PR, what do you suggest are the next steps? Either:

1. There is no way this PR will be included in a codebase. In that case, I will continue using my fork. And we should close this PR.
2. You think this is a reasonable extension of the theme and you are willing to accept the changes after maybe some conditions are met.

[bobthecow]

I know how pull requests work. Part of the process is understanding the motivation behind the pull request, which I'm trying to do. Please take my questions at face value.

I have never used Ranger, and didn't find any documentation in my brief search about spawning shells, or how spawned shells interact with Ranger. Help me understand it, please :)

What does it mean to be a shell spawned from Ranger? How does it affect the environment? How is it useful to show people that their shell was spawned from Ranger? What is Ranger level? How is it useful to show people the level?