Shell script support

[jonnor]

For quick & dirty work, for instance when prototyping, it can be very useful to have a MsgFlo participant based on shell scripts, and mostly calling existing programs.

This can be done fairly simply in Python or Node.js, but one always needs to write some custom code, and this "host" wrapper code obscures the real meat; the commandline/script that is being run.
One could imagine a generic msgflo-script which would take on the commandline.

1. A shell script snippet
2. A string description of the component
3. Standard configuration like role/broker
   It would then always expose an in port, and have outports out and error. The input message payload is provided as stdin. If the script (all commands in it) passes, the anything produced on stdout is send on out. Otherwise, stderr is sent on err.

If it is JSON input, script can for instance use jq to query https://stedolan.github.io/jq/ it

[jonnor]

Should probably also support a --file to take the script, so it can be used with MsgFlo component library, and have a handler for file on form myscript.sh.

[jonnor]

Ideally also support --iips, and transform those into variables or envvars, accessible in the script.