When loading client-id/client-secret/cookie-secret from env variables, google_auth_proxy blows up.

e.g.
```shell
export GOOGLE_AUTH_PROXY_COOKIE_SECRET=foo
export GOOGLE_AUTH_PROXY_CLIENT_SECRET=bar
export GOOGLE_AUTH_PROXY_CLIENT_ID=baz

google_auth_proxy -http-address=127.0.0.1:8443 -upstream=http://127.0.0.1:8080/ -redirect-url=https://foo.bar -google-apps-domain=foo.bar
```

errors out as:
```
panic: runtime error: assignment to entry in nil map

goroutine 16 [running]:
runtime.panic(0x74eac0, 0x98be13)
	/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
main.LoadOptionsFromEnv(0x719100, 0xc20808e000, 0x0)
	/root/gopath/src/github.com/bitly/google_auth_proxy/env_options.go:30 +0x3d1
main.main()
	/root/gopath/src/github.com/bitly/google_auth_proxy/main.go:58 +0x9ff
```