Support changing the Command program

[Popog]

Since Command mostly adheres to a builder pattern, and spawn can be called multiple times, it would be useful to change the program.

Motivating example: a user wants to fall back to an less common filename or path in the case of a failure to spawn.

Proposed signature would look:
pub fn program<S: AsRef<OsStr>>(&mut self, program: S) -> &mut Command

Calling program would imply calling arg0.

It doesn't look like the effected filename has a whole lot of other implications to worry about.

I could put together a PR for this if you're interested.

[tailhook]

Sorry for late reply. I somehow missed the notification. Looks good! Feel free to make PR.