add support for chainable undefined

[marc-portier]

looking at pallets/jinja#997
(as found in https://stackoverflow.com/questions/19614027/jinja2-template-variable-if-none-object-set-a-default-value)

we could be adding the ChainableUndefined to allow for flexible template-writing that introduces _.top.sub | default('if top or sub where not there') in stead of requiring template-writers to be doing lots of protective if exists testing