Locally caching remote or inline resources via "cache" key

As of 1.0.0-beta.15, only one of path, url, or data is allowed on a given resource. In discussing this, it was suggested that it is useful to have a local path to which to download a remote (url) or inline (data) resource so that non-datapackage-aware tooling can access. As a result of discussion in #223, it has been suggested that a new key, cache, be used on a resource to specify a local path to “cache” remote or inline resources to disk.

Examples:

{
  "url": "http://xyz.com/dataset.csv",
  "cache": "dataset.csv"
}

{
  "data": [
    {"a": "1", "b": "2"}
  ],
  "cache": "dataset.csv"
}

So, when a resource has a url or data key, it SHOULD also have a cache key. An implementation that is reading the data in the data package, should then first check cache for the data, if it exists, and, if not, save the data as specified in url or data to cache. Otherwise, cache acts just like path. When a resource has a path key, an implementation should ignore cache.

WDYT? @morty @amercader @rgrp @pwalsh

[rufuspollock]

Removing from v1 milestone as now a pattern item.

[pwalsh]

This was done in #331 and will close when it is merged.