[Propose] Add Stderr.write and Stderr.print

[joshgoebel]

Propose extending the io module with Stderr class and adding:

Stderr.write(s) - Write a string to srderr
Stderr.print(s) - Write a string to stderr followed by a newline

Related: #74

This is towards the goal of making the CLI actually useful for shell scripting. Print errors and returning error codes is critical.