add vtt as subtitle extension

ox/file.py

@@ -24,7 +24,7 @@ EXTENSIONS = {
         'bmp', 'gif', 'jpeg', 'jpg', 'png', 'svg', 'webp'
     ],
     'subtitle': [
-        'idx', 'srt', 'sub'
+        'idx', 'srt', 'sub', 'vtt'
     ],
     'video': [
         '3gp',

ox/vtt.py

@@ -3,7 +3,7 @@
 import codecs
 
 import ox
-
+import srt
 
 def _webvtt_timecode(t):
     return ox.format_duration(t * 1000, years=False)
@@ -30,3 +30,13 @@ def encode(data, webvtt=False):
         )
 
     return codecs.BOM_UTF8 + srt.encode('utf-8')
+
+def load(filename, offset=0):
+    '''Parses vtt file
+
+    filename: path to an vtt file
+    offset (float, seconds): shift all in/out points by offset
+
+    Returns list with dicts that have in, out, value and id
+    '''
+    return srt.load(filename, offset)