[Feature] Show help when args are empty

[xcodebuild]

Run without args:

error: The following required arguments were not provided:
    <CHAR>

USAGE:
    fontfor-macOS [FLAGS] <CHAR>

For more information try --help

Run with --help

fontfor 0.3.0
7sDream <i@7sdre.am>
find fonts which can show a specified character and preview them in terminal or browser

USAGE:
    fontfor-macOS [FLAGS] <CHAR>

FLAGS:
    -h, --help       Prints help information
    -p, --preview    Preview character use supported fonts in browser
    -t, --tui        Enable Terminal UI mode enable this mode will disable the --preview/-p and ignore --verbose/-v arg
    -V, --version    Prints version information
    -v, --verbose    Verbose mode, show all font styles

ARGS:
    <CHAR>    The character

Just show help when args are empty could be better

[7sDream]

This seems can be done easily by using clap's ArgRequiredElseHelp argument.

Would you like to send a PR to do this?