Timeout support

[epage]

In getting feedback, there came up the need for timeout support.

We'd have to run the command in a thread, sleep for the time (in small increments?), and then call kill if needed.

https://doc.rust-lang.org/std/process/struct.Child.html#method.kill

[epage]

Looks like this will be pretty trivial

https://crates.io/crates/wait-timeout

[epage]

Nevermind, it doesn't support getting the output and it doesn't look like it will in the foreseeable future
alexcrichton/wait-timeout#4

[epage]

The responsibility for this is moving over to assert_cmd. This issue will be addressed as assert-rs/assert_cmd#10