Custom component rendering should not have to stub out core methods

[darrenjennings]

Currently, for custom rendering section, you have to implement all the methods that are "core" to the functionality of the custom rendered component. We should handle these methods for you.

Example custom section:
https://github.com/Educents/vue-autosuggest/blob/master/docs/UrlSection.vue

Suggested solution:
Either create mixin with shared methods, or create https://vuejs.org/v2/api/#extends for ability to override methods.