In JS shellCommand should allow stdin & stderr handling

[jasonm23]

shellCommand(command) can be extended to allow for options

Current usage:

stout = shellCommand("command string")

The simplest implementation, add a new function to avoid modification issues.

[stdout, stderr] = shell(command: String, stdin: String?)

[jasonm23]

[stdout, stderr] = shell(command: String, stdin: String?)

Added in 76dc8737ff09eaae5a7577e5c4e3cab6da5faaa9 (90+% covered, uncovered are a couple of safety cases, which will probably never fire in the wild.)