Add Cmd::before_spawn to allow modifying the resulting std::process::Command before spawning it

[swlynch99]

Hey! I've been using this crate a bunch and it's been great!

For one of my use cases I need to run a process under a different uid. If I want to do this I can use xshell to build the command then I need to eject out and convert it directly to std::process::Command to set the uid. If I do that, though, I can no longer use read and I end up needing to write a bunch of code to replicate it.

Adding a uid method felt too platform-specific so this PR instead adds a before_spawn method that registers a function that gets called with the std::process::Command just before it is spawned. It is based on the before_spawn method in duct which does the exact same thing.

Notes

• CmdData needs to be Clone + Send + Sync and Box<dyn Fn(&mut Command)> is none of those so I ended having to place the list of methods directly in Cmd.
• I started off trying to bound the functions by 'a but that broke one of the tests so I've left the hook as Box<dyn Fn(&mut Command) + 'static> instead.
• Hook functions don't get run when converting Cmd to std::process::Command since they are fallible. I have documented this in the method documentation.