Add WebVTT output support

This subset of the format is almost identical to SRT, but I think it's
cleaner to have a separate module (at the cost of a little bit of
copy-pasta).

ox/__init__.py

@@ -13,6 +13,7 @@ from . import jsonc
 from . import net
 from . import srt
 from . import utils
+from . import vtt
 
 from .api import *
 from .file import *

ox/vtt.py

@@ -0,0 +1,32 @@
+# -*- coding: utf-8 -*-
+# vi:si:et:sw=4:sts=4:ts=4
+import codecs
+
+import ox
+
+
+def _webvtt_timecode(t):
+    return ox.format_duration(t * 1000, years=False)
+
+
+def encode(data, webvtt=False):
+    """Encodes subtitles into WebVTT format
+
+    data: list of dicts with 'in', 'out': float and 'value': unicode
+
+    Returns: a UTF-8-encoded bytestring
+
+    >>> encode([{'in': 1.25, 'out': 60 * 60 + 1, 'value': u'touch\\u00E9'}])
+    '\\xef\\xbb\\xbfWEBVTT\\r\\n\\r\\n1\\r\\n00:00:01.250 --> 01:00:01.000\\r\\ntouch\\xc3\\xa9\\r\\n\\r\\n'
+    """
+    srt = u'WEBVTT\r\n\r\n'
+
+    for i, s in enumerate(data, 1):
+        srt += '%d\r\n%s --> %s\r\n%s\r\n\r\n' % (
+            i,
+            _webvtt_timecode(s['in']),
+            _webvtt_timecode(s['out']),
+            s['value'].replace('\n', '\r\n').strip()
+        )
+
+    return codecs.BOM_UTF8 + srt.encode('utf-8')