Warn when commands are not run

```
warning: unused `rustwide::cmd::Command` that must be used
   --> src/docbuilder/rustwide_builder.rs:591:9
    |
591 |         build.cmd("mkdir").args(&["iPhoneOS.platform"]);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
    = note: call `.run()` to run the command
```

Author: jyn514

src/cmd/mod.rs

@@ -188,6 +188,7 @@ impl<'a, B: Runnable> Runnable for &'a B {
 /// output processing, output logging and sandboxing.
 ///
 /// [std]: https://doc.rust-lang.org/std/process/struct.Command.html
+#[must_use = "call `.run()` to run the command"]
 pub struct Command<'w, 'pl> {
     workspace: Option<&'w Workspace>,
     sandbox: Option<SandboxBuilder>,