Allow command to run without config file, and to specify trigger filters with flags

[henrywallace]

Currently, the netwatch command won't run without a config file, which it tries to find by default at ./config.toml.

There exist builtin subscribers that we could use like --builtin log. But which events to filter on? By default we could say all. And then also support ability to specify trigger filters with something like --filter=host.new,port.found.

We probably want to disallow --only flag if the above --builtin or --filter is given, since using the config or not are in conflict with each other.