add configuration

[jacek-kurlit]

What was done?

• Added loading configuration from file at ~/.config/pik/config.toml
• Created AppSettings that hold all settings tui requires (viewport and filtering option for now). Settings are built from config and cli args
• Added logic to override app config with cli args
• Changed default screen size to 25 lines
• Some code refactor
• Added tests

[jacek-kurlit]

@grouzen Would you like to make code review?

[grouzen]

@jacek-kurlit I must say I just started learning Rust, so I can't really spot any non-ideomatic Rust code, but in terms of modularity and logic, it looks good to me! I like the new app settings module!