feat: Use GNU/POSIX-style argument syntax conventions for CLI flags

[timflannagan]

Signed-off-by: timflannagan [email protected]

[timflannagan]

Link to style guide: https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html.

[timflannagan]

Local output after these changes:

$ ./target/release/dcp --help
dcp 0.1.0
exdx
docker cp made easy

USAGE:
    dcp [FLAGS] [OPTIONS] <IMAGE>

FLAGS:
    -h, --help               Prints help information
    -V, --version            Prints version information
    -w, --write-to-stdout    Whether to write to stdout instead of the filesystem

OPTIONS:
    -p, --content-path <CONTENT-PATH>      Where in the container filesystem the content to extract is [default: /]
    -d, --download-path <DOWNLOAD-PATH>    Where the image contents should be saved on the filesystem [default: .]

ARGS:
    <IMAGE>    Container image to extract content from

[timflannagan]

whoops - need to update tests too