add -version option to anim_dump,anim_diff and img2webp

This is to harmonize the -h/-version options on all our examples.

+ added GetAnimatedImageVersions() method to anim_util.*

Change-Id: I2304a1c29e310682e97f236d3867274a192a7a09

examples/anim_util.c

@@ -786,3 +786,9 @@ void GetDiffAndPSNR(const uint8_t rgba1[], const uint8_t rgba2[],
     *psnr = 4.3429448 * log(255. * 255. / sse);
   }
 }
+
+void GetAnimatedImageVersions(int* const decoder_version,
+                              int* const demux_version) {
+  *decoder_version = WebPGetDecoderVersion();
+  *demux_version = WebPGetDemuxVersion();
+}