Add support to add TinyMCE in TextArea

[berlotto]

I'm using flask-mongoengine like follow:

class PostBase(db.Document):
    #many other fields...

class Post(PostBase):
    body = db.StringField(required=True)

class PostModel(model.ModelAdmin):
    list_display = ('title','created_at', 'get_absolute_url')
    # only = ('username',)
    exclude = ('created_at','meta')
    search_fields = ('title', 'created_at')

admin.register(Post, PostModel)