Provide stricter tests

[jwoLondon]

Currently 'tests' consist of working examples that are verified by visual inspection.

A useful addition would be tests that confirm that the json spec produced by a spec-generating function is what is expected. This would also allow documentation examples to be tested through elm-verify-examples.

We might also consider validating against Vega-Lite and Vega schemas, although there are still some cases where working VL/V examples can invalidate schemas.

[kachkaev]

Perhaps, using elm-test could be a solution – the package looks rather straightforward to get started with. I have some experience with setting up Travis CI for the github repos, so can help a bit here.

[kachkaev]

elm-css uses elm-test and might be a good source of inspiration.