vidjilparser.py make file output optional

since we may not want to export to a file, leaving the filepath variable
at None will disable the file writing capabilities.

tools/vidjilparser.py

@@ -4,28 +4,31 @@ from six import string_types
 
 class VidjilWriter(object):
 
-    def __init__(self, filepath, pretty=False):
+    def __init__(self, filepath=None, pretty=False):
         self._filepath = filepath
         self.pretty = pretty
         self.buffer = ""
         self.buffering = False
         self.conserveBuffer = False
+        self.file = None
 
     def __enter__(self):
-        self.file = open(self._filepath, 'w')
+        if self._filepath:
+            self.file = open(self._filepath, 'w')
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
-        self.file.close()
+        if self.file:
+            self.file.close()
 
     def write(self, prefix, event, value, previous):
         res = self._write(prefix, event, value, previous)
         if self.buffering:
             self.buffer += res
             return ""
-        else:
+        elif self.file:
             self.file.write(res)
-            return res
+        return res
 
     def _write(self, prefix, event, value, previous):
         if self.pretty:
@@ -67,7 +70,8 @@ class VidjilWriter(object):
 
     def writeBuffer(self):
         try:
-            self.file.write(self.buffer)
+            if self.file:
+                self.file.write(self.buffer)
             return self.buffer
         finally:
             self.purgeBuffer()
@@ -106,7 +110,7 @@ class VidjilParser(object):
         if writer is not None:
             self._writer = writer
         else:
-            self._writer = VidjilWriter('out.json')
+            self._writer = VidjilWriter()
         self._model_prefixes = []
         self.prefixes = []