added file:write() & an optional mode param to os.open()

examples/reader.cosmo

@@ -1,9 +1,6 @@
-
 local err, file = os.open("LICENSE.md")
-print("made file")
 if err then
   print("failed to open file")
 end
 
-print(file)
 print(file:read("a"))

examples/writer.cosmo

@@ -0,0 +1,6 @@
+local err, file = os.open("test.md", "w")
+if err then
+  print("failed to open file")
+end
+
+file:write("hello world")