Implement link checker and other server side validation on Travis

[jmatsushita]

Feeding into #7 implementing a server side link checker would be a good PoC. What would be even better is an isomorphic link checker.

[jmatsushita]

Validating keys for SPAR standard compliance would be another client side/server side validator need. #12

[jmatsushita]

Another possible CI test that would be useful if the management of tags is open, it provide feedback on sparseness of tags to help improve the usability and relevance of more fluidly defined and evolving taxonomies.

[jmatsushita]

@gembarrett Would love to hear your thoughts on how to include style guide as a validator!

[jmatsushita]

Given how Travis actually only shows a canned The Travis CI build failed message on pull requests via the Commit Status API, it might be better (and more in line with the overall micro-service approach) to develop (or wrap or automate) our own CI service this would help in being able to do browser based tests (although might still need some isomorphic smarts or local deployment to deal with offline use).

[jmatsushita]

It's working on the init project!

[jmatsushita]

The ProseMirror linting demo is awesome and MarkdownLint is interesting.