Allow ExIrc.Client.start_link to use process opts

Prior to this commit, registering the name for an ExIrc.Client process,
it was a two-step process of ExIrc.Client.start_link and then
Process.register. This also made registering a name for
supervisor-started ExIrc.Client processes a bit tricky. This commit
allows end users to start ExIrc.Client processes from a supervisor,
but still register a name, as in the example below.

children = [
  worker(ExIrc.Client, [[debug: true], [name: :irc_client]])
  ]
supervise(children, strategy: :one_for_all)