Add flatpak flag

[lil5]

For example use -F as the argument to identify flatpak commands

Here is a list of usage examples:

• -Fu -> flatpak update
• -Fr -> flatpak uninstall
• -F <name> -> flatpak install <name>
• -Fi <name> -> flatpak info <name>
• -Fs <glob> -> flatpak search <glob>

[fosskers]

I will consider this, thank you for the suggestion.