Custom model field

It would be nice to specify Markdown in the model:

```
from django_markdown.fields import MarkdownField

class Item(models.Model):
...
md = MarkdownField()
```
##

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Start by inspecting the repository's existing Django integration and any model-field implementations or tests. The work is complete when users can import django_markdown.fields.MarkdownField and declare it on a Django model as shown, with the expected Markdown behavior covered by the project's tests.

Written by the indexing model from the issue text.