Add --cargo-opt= option for passing global options to cargo

Closes rustwasm#923

src/command/mod.rs

@@ -137,7 +137,9 @@ impl GlobalOpts {
     /// The caller should use `.args` to specify the cargo subcommand,
     /// and set the current directory, and so on.
     pub fn cargo_cmd(&self) -> std::process::Command {
-        std::process::Command::new("cargo")
+        let mut cmd = std::process::Command::new("cargo");
+        cmd.args(&self.cargo_opt);
+        cmd
     }
 }
 

src/lib.rs

@@ -77,4 +77,9 @@ pub struct Cli {
 #[derive(Debug, StructOpt)]
 /// Global options, passed to every subcommand.
 pub struct GlobalOpts {
+    /// Additional global argument to pass to `cargo`, before the cargo subcommand.
+    /// For example, `--cargo-opt=--locked`.
+    /// May be repeated to pass multiple options.
+    #[structopt(long="cargo-opt", number_of_values = 1)]
+    pub cargo_opt: Vec<String>,
 }