hanlde custom elements

[nbriz]

right now there are methods for adding custom elements or attributes to the netitor's internal database:

ne.addCustomElements(obj)
ne.addCustomAttributes(obj)

but we should also include some way for the netitor to avoid throwing errors on custom elements that have been imported/registered, maybe this is a good place to start: https://stackoverflow.com/questions/27334365/how-to-get-list-of-registered-custom-elements