Add an option to get all violations after one validation

[Gnomus042]

I am trying to make something similar to the Structured Data Testing Tool (https://search.google.com/structured-data/testing-tool/u/0/) with ShEx using shex.js. In the tool, it's possible for the user to see all violations at once, but I can't do the same with shex.js (In some cases it stops after finding severe issues and doesn't return MissingProperties, for example)