pass arg to include for limiting selection

[Daniel Skinner]

it might be helpful to be able to pass an arg to builtin include that specifies which top-level blocks to include, ignoring the rest.

This stems from noticing a need to abstract out certain reusable blocks into seperate files for reuse in many places, but the creation of so many files with little bits of information creates confusion. Typically, it seems there is a particular page that outlines the majority of these blocks for use there, and reuse elsewhere. Workflow might be simplified with this addition as well as keeping the original work contained in its more intended usage, and removing the file cruft.