Removes missing screenshots from README for kube-ps1 plugin.
Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

Fixes #8786

Closes #8009
Co-authored-by: Jon Mosco <jonny.mosco@gmail.com>

I ran into this issue, looks like maybe a few other people did as well.

https://github.com/jonmosco/kube-ps1/issues/28#issuecomment-359761463
A simple clarification seems helpful.

changes:
- easily customize the colors via variables
- command to switch on/off the custom prompt
- Improved documentation with example on how to append on the prompt.

Fixes #7261

* use https everywhere

* use https links on the files that are left

Also, removed some broken links and updated redirections.