Fix several issues

[tbarusseau]

• Cargo.toml can contain dependencies with non-semver versions. This is an issue because cargo build : requires a semver version. Fix: added cargo metadata parsing to get a semver version
• Added: --skip-update flag, as you don't always want to run cargo update
• Fixed: Can now parse complex Cargo.toml entries, like dep: { version: "0.0.1", ... }
• Changed: The logging is now prefixed by 4 spaces to match cargo's log output