request: switch to easily disable all default blocks

[draeath]

For my own use case, I just want to get rid of a very specific and small number of parameters (for example the highlight=<term> parameter that Sphinx adds to search results on the official Python documentation pages.

I realize I can copy the defaults into the override field, but I have to worry about maintaining that list.

It would be nice if there was a switch that would just disable the defaults entirely without having to copy that over periodically.