feat(api): add &mut Command to pre_spawn()

This addition makes it possible to mutate the `Command` in
`pre_spawn()`, spawn a `Child` from that mutated command, then roll-back
the changes to command in `post_spawn()`.

Author: TheLostLambda

src/generic_wrap.rs

@@ -91,7 +91,7 @@ macro_rules! Wrap {
 				for (id, wrapper) in wrappers.iter_mut() {
 					#[cfg(feature = "tracing")]
 					::tracing::debug!(?id, "post_spawn");
-					wrapper.post_spawn(&mut child, self)?;
+					wrapper.post_spawn(command, &mut child, self)?;
 				}
 
 				let mut child = Box::new(
@@ -204,7 +204,7 @@ macro_rules! Wrap {
 			/// how `CreationFlags` on Windows works along with `JobObject`.
 			///
 			/// Default: no-op.
-			fn post_spawn(&mut self, _child: &mut $child, _core: &$name) -> Result<()> {
+			fn post_spawn(&mut self, _command: &mut $command, _child: &mut $child, _core: &$name) -> Result<()> {
 				Ok(())
 			}