Allow icon overrides using cli args and formalize flag names

[Anupal]

Introduce icon flags to use custom icons. Basic idea is that not all machines will have Nerdfonts installed or users might want customization.

CPU

• Use flags to display a cpu and percent icons. Same flags can be used to provide overrides.
• By default, icons are hidden

$ stmx cpu --help
Description:
  get current cpu usage

Usage:
  stmx cpu [options]

Options:
  -i, --show-icon <show-icon>        show icon (optionally provide custom icon)
  -p, --show-percent <show-percent>  show percent icon (optionally provide custom icon)
  -?, -h, --help                     Show help and usage information

Memory

• Use flags to display a memory and percent icons. Same flags can be used to provide overrides.
• Percent is now a unit type and also the default value.
• By default, icons are hidden

$ stmx memory --help
Description:
  get current memory usage

Usage:
  stmx memory [options]

Options:
  -i, --show-icon <show-icon>                                                                                 show icon (optionally provide custom icon)
  -p, --show-percent <show-percent>                                                                           show percent icon (optionally provide custom icon)
  -u, --unit <Bytes|GibiBytes|GigaBytes|KibiBytes|KiloBytes|MegaBytes|MibiBytes|Percent|TeraBytes|TibiBytes>  Select the memory unit for display [default: Percent]
  -?, -h, --help                                                                                              Show help and usage information

Battery

• Use flags to display battery and percent icons.
• Percent flag can be used to provide an override.
• I won't support override for battery and charging yet. It will be implemented as a separate feature.
• By default, icons are hidden

$ stmx battery --help
Description:
  get current battery status

Usage:
  stmx battery [options]

Options:
  -i, --show-icon                    show battery capacity icon
  -s, --show-status-icon             show battery status icon
  -p, --show-percent <show-percent>  show percent icon (optionally provide a custom icon)
  -?, -h, --help                     Show help and usage information