The usage pattern: Imagine  we have a client which subsribed to multiple topics
inside one connection. It track changes of specific items state by their ID
as example.

The test is passing in general case. But run it with race detection option and
it fails due data race condition detected.

==================
WARNING: DATA RACE
Write at 0x00c0000120d8 by goroutine 39:
  github.com/alexandrevicenzi/go-sse.(*Channel).SendMessage()
      /Users/iscander/projects/go-sse/channel.go:26 +0x8c
  github.com/alexandrevicenzi/go-sse.(*Server).SendMessage()
      /Users/iscander/projects/go-sse/sse.go:116 +0x21b
  github.com/alexandrevicenzi/go-sse.TestMultipleTopics.func2()
      /Users/iscander/projects/go-sse/sse_test.go:135 +0xce

Previous write at 0x00c0000120d8 by goroutine 38:
  github.com/alexandrevicenzi/go-sse.(*Channel).SendMessage()
      /Users/iscander/projects/go-sse/channel.go:26 +0x8c
  github.com/alexandrevicenzi/go-sse.(*Server).SendMessage()
      /Users/iscander/projects/go-sse/sse.go:116 +0x21b
  github.com/alexandrevicenzi/go-sse.TestMultipleTopics.func2()
      /Users/iscander/projects/go-sse/sse_test.go:135 +0xce

Goroutine 39 (running) created at:
  github.com/alexandrevicenzi/go-sse.TestMultipleTopics()
      /Users/iscander/projects/go-sse/sse_test.go:132 +0x735
  testing.tRunner()
      /usr/local/Cellar/go/1.16.3/libexec/src/testing/testing.go:1193 +0x202

Goroutine 38 (running) created at:
  github.com/alexandrevicenzi/go-sse.TestMultipleTopics()
      /Users/iscander/projects/go-sse/sse_test.go:132 +0x735
  testing.tRunner()
      /usr/local/Cellar/go/1.16.3/libexec/src/testing/testing.go:1193 +0x202

Protect the option by wrappring into a mutex. The datarace is gone.
It closes alexandrevicenzi#18