Runtime configuration reloading

[stephank]

We now support listfiles for various settings (allowed_origins, allowed_domains, blocked_domains), which is especially handy for longer, dynamic lists. It'd be useful if we could reload dynamic lists at runtime, such as when using an external blocklist.

We can probably bunch this together with reloading more configuration settings, though not necessarily everything. For example, it's pretty common for other software not to allow runtime rebinding the listening socket, and we should draw a similar line somewhere to prevent complexity.

cc @jimdigriz, because you mentioned the idea in #96, but feel free to unsub otherwise. 🙂