Invoke-Process

Invoke-Process is a simple wrapper function that aims to "PowerShellyify" launching typical external processes. There
are lots of ways to invoke processes in PowerShell with Start-Process, Invoke-Expression, & and others but none account
well for the various streams and exit codes that an external process returns. Also, it's hard to write good tests
when launching external proceses.

This function ensures any errors are sent to the error stream, standard output is sent via the Output stream and any
time the process returns an exit code other than 0, treat it as an error.