add custom style support

[wuyue92tree]

custom style from attrs when using JSONEditor

# admin.py
class MyCustomAdmin(admin.ModelAdmin):
    formfield_overrides = {
        JSONField: {'widget': JSONEditor(attrs={'style': 'width: 620px;'})}
    }

in django admin

for #61

[nnseva]

Hi @wuyue92tree ,

Thank you for your excellent contribution.

Would you please to add a description of the option you've created, to the README.md file, to make it available for other users?

[wuyue92tree]

@nnseva Usage has been added to README.md 😄