Upgrade to clap for argument parsing

As mentioned on #28, command line arguments could use an overhaul, so kicking it off with an upgrade to `clap` since `gumdrop` seems to be no longer maintained. I set this up to break as little as possible existing behaviour, if we are willing to change the version flag to clap default `-V` from `-v` it removes the need to have a version arg also. Output of `cargo-aur --help` - Old ``` Usage: /home/nickd/.cargo/bin/cargo-aur [OPTIONS] Positional arguments: free Absorbs any extra junk arguments. Optional arguments: -h, --help Display this help message. -v, --version Display the current version of this software. -o, --output OUTPUT Set a custom output directory (default: target/). -m, --musl Use the MUSL build target to produce a static binary. -d, --dryrun Don't actually build anything. ``` Output of `cargo-aur --help` - New ``` Prepare Rust projects to be released on the Arch Linux User Repository. Usage: cargo-aur [OPTIONS] Options: -o, --output <OUTPUT> Set a custom output directory (default: target/) -m, --musl Use the MUSL build target to produce a static binary -d, --dryrun Don't actually build anything -v, --version Print version -h, --help Print help ```