Consider adding a single-header support

[kamchatka-volcano]

This has already been tested with https://github.com/vinniefalco/Amalgamate and it seems to work fine.

A change to includes format may be required, and there should be an unit tests configuration using a single header. CI configurations for running amalgamation, testing it, and publishing a single header on each new release should also be created.