gum file - support --header

[lbdyck]

Is your feature request related to a problem? Please describe.
When using gum file I would like to provide instructions on how/what to select but there is no --header option. Using echo somewhat works but not as nice and using gum style also is not perfect.

Describe the solution you'd like
Support --header along with related color (foreground/background) options on gum file

Describe alternatives you've considered
Tried echo and gum style with minimal success.

Additional context
LOVE gum

[jamesbvaughan]

This would be great! A useful part of this behavior for me would be to have the header disappear after a file has been chosen, or at least to have an option to enable that behavior.

[caarlos0]

see #768