Changes

• Use double quotes for show_usage function: termux-battery-status.in
• Colorize show_usage text: termux-battery-status.in
• Provide valid options when unknown is passed: termux-battery-status.in
• Specify script type for ShellCheck to get rid of its complaints: all scripts

Questions

• I wonder if it's desirable to color help messages or not.
• Maybe it's a good idea to use gum instead of raw escape sequences for colors, isn't it? But in this case it's required (from my understanding) to preinstall gum too.