Fix scheming command to allow dataset_fields key.

[brew]

Group and Org schemas use fields as the key to the array within the
schema json file. Dataset schemas use dataset_fields. This commit
stops the scheming command from breaking when printing dataset fields.

[brew]

Go on then ;)

[wardi]

If I'm reading this correctly this will separate the resource fields from their corresponding dataset fields when multiple dataset schemas are enabled. That might be confusing. Can you instead list the resource fields right after the dataset fields for a given schema?

[brew]

It treats the resource schema like a separate schema. The output is after datasets, like this:

Dataset schemas:
 * "dataset"
   - "fluent_title"
   - "name"
   - ...

Resource schemas:
 * "resource"
   - "url"
   - "fluent_name"
   - "fluent_description"
   - "format"

Group schemas:
 * "group"
   - "title"
   - "name"
   - "notes"
   - "url"
   - "bookface"

Organization schemas:
    plugin not loaded or schema not specified

But, yes, we could have it so resources appear with "dataset" like this:

Dataset schemas:
 * "dataset"
   - "fluent_title"
   - "name"
   - ...

 * "resource"
   - "url"
   - "fluent_name"
   - "fluent_description"
   - "format"
...