Add preselected options when using filter command

[sestegra]

Is your feature request related to a problem? Please describe.
I'm using gum to let the user to select from a list of options with some of them are defaults.
To ease the selection, I need to have a list of preselected options using filter command.

Describe the solution you'd like
Add --selected flag when using gum filter command to preselect options.

Describe alternatives you've considered
No alternative found yet.

Additional context
N/A