Shrink on commands' parameters

Changelog:

• shrink on parameters of commands @ 2ea4163
• fix not working toString methods @ f1ffeff
• update and clean provided examples
• update and clean README

Provide a faster shrinker

Changelog:

• Provide a faster commands shrinker @ edcb4d3

Fix shrinker and provide better counterexamples

Changelog:

• fix commands shrinker: shrinker considered all commands as unlaunched so was unable to provide jsverify with useful shrinks @ c376791
• remove unlaunched from the counterexample @ 6e141b6

Update README

No functional changes added.
Provide a better README for npm package manager.

Initial release

Extract jsverify-commands from https://github.com/dubzzz/scala-2048 project

It comes with 3 functions:

• command
• commands
• forall