Prettier help

Aligns all right column descriptions to be equal.

**before:**

```
$ go run -- examples/greet-with/main.go hello to -h
Usage:
  main [options...] <greeting> to <greetee>

Say a greeting to a specific persion

Example:
  $ greet-with hello to jerry
  hello jerry

Options:
  -h, --help  # show help and exit

Options for `main`:
  -c, --color="blue"  # color the output (red, blue, green)
  -h, --help          # show help and exit
  -l, --loudly        # say loudly
  -v, --version       # show version and exit
```

**after:**
```
$ go run -- examples/greet-with/main.go hello to -h
Usage:
  main [options...] <greeting> to <greetee>

Say a greeting to a specific persion

Example:
  $ greet-with hello to jerry
  hello jerry

Options:
  -h, --help           : show help and exit

Options for `main`:
  -h, --help           : show help and exit
  -v, --version        : show version and exit
  -l, --loudly         : say loudly
  -c, --color=blue     : color the output (red, blue, green)
```