add indicator style

[nim65s]

Hi,

I saw a "good first issue" on #857, so I gave it a try :)

TODO

• [x] Use cargo fmt
• [x] Add necessary tests
• [x] Add changelog entry
• [x] Update default config/theme in README (if applicable)
• [x] Update man page at lsd/doc/lsd.md (if applicable)

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[nim65s]

NB: I tried to keep compatibility with previous options / config files, but it might be cleaner to allow some refactoring here