Improve README

Author: rikyoz

README.md

@@ -16,13 +16,13 @@
 
 ## Screenshots
 <p align="center">
-<a href="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot0.png">
+<a href="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot0.png" target="_blank">
 <img src="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot0.thumb.png" />
 </a>
-<a href="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot1.png">
+<a href="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot1.png" target="_blank">
 <img src="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot1.thumb.png" />
 </a>
-<a href="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot1.png">
+<a href="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot1.png" target="_blank">
 <img src="https://raw.github.com/rikyoz/mrhash/screenshots/doc/img/v0.3.2/screenshot2.thumb.png" />
 </a>
 </p>