Support plain definition $ref

Currently we need ` $ref: '#/definitions/Color'` and we should support `$ref: Color`

```yaml
Palette:
type: object
properties:
palette_name:
type: array
items:
$ref: '#/definitions/Color'
Color:
type: string
```

The idea is to support

```yaml
Palette:
type: object
properties:
palette_name:
type: array
items:
$ref: Color
Color:
type: string
```

Internally flasgger should transform `$ref: Color` in to ` $ref: '#/definitions/Color'` in the json output.

No files or tests are named. Start by tracing how Flasgger reads schema references and produces JSON output, then identify the normalization point for `$ref` values; done means `$ref: Color` is emitted as `#/definitions/Color` while the existing form continues to work.

Written by the indexing model from the issue text.