RFE: Expose exit code from Terminal's shell #72

[schnoddelbotz]

As discussed in issue #72

• Don't panic if ExitCode() is called before RunLocalShell() - return -1 like os.exec would before wait()
• Add a minimal test for the above
• Add windows support
• Update documentation/README with example usage

[andydotxyz]

Good implementation thanks. Just a note about the new API return info.

[andydotxyz]

Thanks so much, great change :)