Add the option to change labels and help_text

[michi88]

Not sure if anyone is maintaining this but if so and you want to pull this I can add a description to the readme on how to use it.

What you can do is change the label of the json key field.

data = forms.CharField(widget=SplitJSONWidget(
        attrs=attrs,
        key_field_conf_map={'a': {'label': 'Nice label for a', 'help_text': 'some help text for the a field'}})
    )

Pretty useful as this keeps your json keys clean.