Add command execution utility

[dexterbg]

Meant to simplify using the lib for standard users / scripting / home automation applications. OVMS manual page covering this and general command execution APIs is in the work.

Regards,
Michael

[birkir]

Thats great, It would be way more beneficial if we can add it to package.json/bin so its executable.
Here is the checklist: https://gist.github.com/mritzco/4495cf5fa863779ed1ac0e4edf9a1ed0

# package.json
{
  "bin": "./bin/ovms.js"
}

And if we allow input arguments also, we could simply do:

npx ovms-client -h localhost -vid DEMO -p DEMO stat

Would you rather merge this now and open another one later for this, or complete the task in this PR?