`spit-appender` hard-codes "\n"

[jimpil]

Instead it can simply use system-newline from the same namespace.

On a slightly different note, it seems that spit-appender actually tries to be something like a file-line-appender. If that's the case a slightly cheaper implementation (skips two calls to str) would be the following:

(with-open [^BufferedWriter wrt (io/writer fname :append append?)]
  (let [^String out (force output_)]
    (.write wrt out)
    (.newLine wrt)))

Furthermore, extending the above to use a shared Lock is pretty trivial too (for thread-safety mentioned in another ticket).

[ptaoussanis]

Fixing shortly