Clean up usage of #[derive(Parser)]

[egregius313]

Mostly cosmetic changes. Provides clean up of the usage of #[derive(Parser)]
in the following ways:

1. Makes all attributes of Opts use the comment syntax for defining the help
   message.

2. Uses the short form of the short and long metadata declarations.
   #[derive(Parser)] treats

   struct Example {
       #[clap(short, long)]
       arg: u8,
   }

   the same as

   struct Example {
       #[clap(short = 'a', long = "arg")]
       arg: u8,
   }

[CleanCut]

TIL!

[CleanCut]

Released in v0.3.2