Validation using `qsv schema` and `validate` commands

[jqnatividad]

With 0.11, we already validate and normalize the CSV during analysis.

Will need to add schema support, perhaps, by looking for a schema.json attribute in the package?

If that's set, it will use the designated schema (may it be another resource in the CKAN instance, or a schema.json file on a URL) to validate the resource before pushing it.

[jqnatividad]

Somewhat duplicative of #18 which will store the data dictionary as JSON Schema, primarily, so we can optionally use qsv validate to validate future updates to the resource.