Add webvtt1.vim

This syntax file is for WebVTT: The Web Video Text Tracks Format.

This resolves part of #55. `:past` and `:future` is not support, because
they are used only in parentheses of `cue()`, and current CSS syntax
does not support a highlighting in parentheses of pseudo functions.

test/test.html

@@ -951,6 +951,14 @@
 }
 </style>
 <style>
+.webvtt1::cue,
+.webvtt1::cue-region,
+.webvtt1::cue(#cue1),
+.webvtt1::cue-region(#scroll), {
+  display: auto;
+}
+</style>
+<style>
 .will-change-1 {
   will-change: scroll-position;
 }