Support a custom model field

This allows you to directly tie the CodeMirorTextarea widget to a model field, which is helpful when you want to associate the widget from your model.

Example:
```python
from codemirror.fields import CodeMirrorField

class PostChangeHook(models.Model):
    name = models.CharField(max_length=200,)
    function = CodeMirrorField(max_length=3000,)
```