[#195] Logstash appender: add `:flush?` option (@tvanhens)

Setting the value to true will eagerly flush the tcp writer. Default
behavior is for this to be false.

src/taoensso/timbre/appenders/3rd_party/logstash.clj

@@ -50,9 +50,11 @@
   format to the logstash server at `host:port`. Additionally `opts`
   may be a map with `:pr-stracktrace` mapped to a function taking an
   exception, which should write the stacktrace of that exception to
-  `*out`."
+  `*out`. Set `:flush?` to true to flush the writer after every
+  event."
   [host port & [opts]]
   (let [conn   (atom nil)
+        flush? (or (:flush? opts) false)
         nl     "\n"]
     {:enabled?   true
      :async?     false
@@ -69,6 +71,7 @@
            (locking sock
              (data->json-stream data out (select-keys opts [:pr-stacktrace]))
              ;; logstash tcp input plugin: "each event is assumed to be one line of text".
-             (.write ^java.io.Writer out nl)))
+             (.write ^java.io.Writer out nl)
+             (when flush? (.flush ^java.io.Writer out))))
          (catch java.io.IOException _
            nil)))}))