Add support for configuration via file/stdin

Nobody has claimed this yet.

As the daemon grows our configuration flag options are likely to become cumbersome pretty quickly. I was talking with @bigs and @vasco-santos about needing to add support for specifying the multiaddrs the daemons libp2p node listens on. We could, and probably should, add a comma delimited string flag similar to how we add bootstrap addresses. Over time though, reading through all of the flags you have enabled for a node could get overwhelming.

I'd like to propose adding support for piping a config file to the daemon. For example:
```bash
$ cat config.json | p2p daemon
```

This would also have the nice benefit of being able to have node type configurations pre made for users; DHT booster, relay, rendezvous, etc.

If this sounds good, I can write up a spec for the json configuration.

The issue names the p2p daemon command, configuration flags, and bootstrap-address handling, but no files or tests. Start by locating the daemon entry point and current flag parsing, then determine the configuration format and stdin behavior that need specification. Done requires an agreed config schema and support for piping it into the daemon, with coverage for the resulting configuration.

Written by the indexing model from the issue text.