TLS is (I believe) broken

First, the new `tls_key` and `tls_cert` support is broken in the Set method:

https://github.com/bitly/go-nsq/blob/e0b1455c6272a9c95bad4a020170846644b992a7/config.go#L332

These cases do not return early, like the other switch cases. You don't do this because of the following if block to load certs if both the tls_key and tls_cert flag are not empty:

https://github.com/bitly/go-nsq/blob/e0b1455c6272a9c95bad4a020170846644b992a7/config.go#L360-L366

One problem is that, no matter what, the first pass through this method (as the config is parsing all opts) *necessarily* will not have both `t.certFile` and `t.keyFile` set and will skip the if condition and return `unknown option`. The second is that, even if both were loaded, the if condition doesn't return, at which point the method would still return `unknown option`.

The fix is to make the `unknown option` error return as `default` in the switch statement, and `return nil` as the last line in that method. I would do this myself, however, after testing it, using tls still panics:

```
goroutine 24 [running]:
runtime.panic(0x71e9c0, 0x926eb3)
	/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
github.com/bitly/go-nsq.(*Conn).Close(0x0, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/conn.go:175 +0xcb
github.com/bitly/go-nsq.(*Producer).close(0xc208102000)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/producer.go:239 +0x88
github.com/bitly/go-nsq.(*Producer).onConnIOError(0xc208102000, 0xc2080bcd80, 0x7f13c7b59da0, 0xc208100240)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/producer.go:327 +0x27
github.com/bitly/go-nsq.(*producerConnDelegate).OnIOError(0xc208032138, 0xc2080bcd80, 0x7f13c7b59da0, 0xc208100240)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/delegates.go:141 +0x48
github.com/bitly/go-nsq.(*Conn).WriteCommand(0xc2080bcd80, 0xc2080d1c70, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/conn.go:254 +0x1e7
github.com/bitly/go-nsq.(*Conn).identify(0xc2080bcd80, 0x920248, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/conn.go:293 +0xa1e
github.com/bitly/go-nsq.(*Conn).Connect(0xc2080bcd80, 0x9, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/conn.go:149 +0x346
github.com/bitly/go-nsq.(*Producer).connect(0xc208102000, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/producer.go:220 +0x3f4
github.com/bitly/go-nsq.(*Producer).sendCommandAsync(0xc208102000, 0xc2080d1b30, 0xc20811bc80, 0x0, 0x0, 0x0, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/producer.go:182 +0xd7
github.com/bitly/go-nsq.(*Producer).sendCommand(0xc208102000, 0xc2080d1b30, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/producer.go:165 +0x78
github.com/bitly/go-nsq.(*Producer).Publish(0xc208102000, 0x778bd0, 0x9, 0xc2080bca20, 0x105, 0x116, 0x0, 0x0)
	/mnt/hgfs/twmb/dev/go/src/github.com/bitly/go-nsq/producer.go:150 +0x16f
...
```

Which doesn't happen when I do not use any tls flag (and turn off require-verify with all tls flags in nsqd).