Add a helper for initializing fsStdOut in threads

faststreams/stdout.nim

@@ -1,5 +1,13 @@
 import
   outputs
 
-let fsStdOut* {.threadvar.} = fileOutput(system.stdout)
+var fsStdOut* {.threadvar.}: OutputStream
+
+proc initFsStdOut* =
+  ## This proc must be called in each thread where
+  ## the `fsStdOut` variable will be used.
+  if fsStdOut == nil:
+    fsStdOut = fileOutput(system.stdout)
+
+initFsStdOut()