modularize domaincheck

[jschauma]

It'd be useful to have the logic of the tool in a module, so consumers can call it without having to run the CLI tool (see e.g., intelowlproject/IntelOwl#1682 (comment)). This shouldn't be too much work; we basically have all functionality in functions already, and the main loop can be kept as is for a standalone CLI tool consuming the module.