Allow to launch other programs

[suve]

It would immensely increase the language's usefulness if there was an :exec function (or maybe it should be an !exec construct?) available that'd make it possible to call other programs.

Things to consider:

• Syntax for passing arguments to called programs.
• Syntax for passing input (stdin) to called programs.
• How to return both the exit status and output (stdout) to the user?
• Should there be a facility to retrieve the error output (stderr) of called program?
• Should there be a facility to retrieve the running time of the called program?