Finish `qri validate` command & API endpoint

[b5]

Once we've landed our json schema refactors, we should round out a qri validate command with the following features:

• Should work on existing datasets, listing errors so users can fix them.
• Should Accept arguments for schema and data, allowing qri validate to function on data that isn't checked into version control (useful for checking a dataset before running qri add)
• Pretty, concise output.

I think output should resemble the output of Unit Tests. Listing errors & their locations in the document in red. Mocha Reporters are a great starting point for seeing different styles of error reporting. Validate should give a satisfying green check when no errors, and a readable, usable list of red errors otherwise.

We should also get this to work on the API side, via a POST to /validate.