check isatty for underline in --header

[GnikDroy]

Fixes #775

The isatty check code is copied from core.rs

This PR was drafted very quickly with ripgrep and minimal reading. Hence, it might not be the ideal way to do things. Let me know if you want anything to be changed.

---

TODO

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

[codecov-commenter]

Codecov Report

Merging #777 (bee7627) into master (f5ee0a2) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
- Coverage   86.68%   86.66%   -0.02%     
==========================================
  Files          44       44              
  Lines        4332     4334       +2     
==========================================
+ Hits         3755     3756       +1     
- Misses        577      578       +1     

Impacted Files	Coverage Δ
src/display.rs	84.02% <60.00%> (-0.18%)	⬇️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[meain]

I think it might be better to control it here by changing Header to take more states just to keep it consistent. We should retain the external cli interface the same though.

[GnikDroy]

Okay, I will take a look at this later in the day.