Runnable command-line version of ksc in JS
Original issue: https://github.com/kaitai-io/kaitai_struct/issues/1149

> Given the idea to run tests in JS compiler too (at least to make sure that their output is equal to JVM version), we'll need a quick way to run JS compiler from command line.

> I propose to create a quick wrapper script to run JS compiler akin to ksc - i.e. doing proper CLI parsing.

Current status: actually, we can use almost everything, including the our CLI parser in ScalaJs environment (i.e. [scopt](https://github.com/scopt/scopt) — since ~May 2017 it supports ScalaJS too). The only problem is YAML parser: nowadays JVM and JS builds use different language-native parsers.

Two possible solutions:

* Wrap call of external JS-based YAML library into ScalaJS → lame, but relatively quick hack
* Write our own YAML library in pure Scala, compilable transparently for both JVM and JS (ideally, for native as well) → harder, but offers lots of other good stuff too (like line/char-accurate error messaging)

After that it should be possible to run ScalaJs version of ksc (in nodejs or similar engine) from CLI.