Implement std::io::Write

[heaths]

It would be great if Term implemented std::io::Write so that we can pass it to write!() and other functions that take a Write. Currently, Term::write_line() is all that exists, and I don't always want to write a new line. Pre-formatting strings is a bit of a hassle, but having to always write a line also means we can't really do prompts like:

Please enter some input: <cursor>