Add shorthand letter to flags

[sylviamoss]

Add shorthand options to flags. For example,

diceware-cli generate --size=4 --copy  

It could be shortened to:

diceware-cli generate -s=4 -c

To achieve that, the Flags method should be updated to use the suffixed P version.

--  generateCmd.Flags().StringVar(&dicewareConfig.Lang, "lang", "en", "password language\n available langs: en, pt")
++ generateCmd.Flags().StringVarP(&dicewareConfig.Lang, "lang", "l", "en", "password language\n available langs: en, pt")