Support 'from source' packaging, and using crates.io as a source

Added new flags `--no-bin` and `--source`. Existing behaviour is `--source project`. # New options ## `--source crates-io --no-bin` Download crate from crates.io, get it's checksum, and write a PKGBUILD that downloads and builds from crates.io. No tarball is produced. ### Example PKGBUILD https://gist.github.com/nick42d/6ce7809a0b24444aaccf9655811cb750 ## `--source crates-io` Download crate from crates.io, and create a binary release tarball in `target/cargo-aur`. Write a PKGBUILD that assumes user will upload binary release tarball to github/gitlab ### Example PKGBUILD https://gist.github.com/nick42d/dfe9e652473370671fecae9bce2d84fd ## `--source project --no-bin` Package the source code of the project to a source tarball in `target/cargo-aur`. Write a PKGBUILD that assumes user will upload source tarball to github/gitlab ### Example PKGBUILD https://gist.github.com/nick42d/e441adb325c01cbbee68c835bfe58e3b This change should be non-breaking as `--source` defaults to `project` so I put this as a minor version bump. However, you could consider a major version bump due to the new functionality. Testing note: the version number in Cargo.toml is used to download the .crate. If you try and run `--source crates.io` with `version = "1.7.2"` this will fail since 1.7.2 isn't uploaded to crates.io yet. Change version to 1.7.1 to test the new flags.