Add .piskel extension

Piskel is a browser-based sprite editor. Its file format is JSON-based
(though the JSON contains base64-encoded PNG data).

Recognise the .piskel extension and identify it analogously to .svg: it
is text, image, json and piskel.

https://github.com/piskelapp/piskel
http://justsolve.archiveteam.org/wiki/Piskel_canvas

Author: wjt

identify/extensions.py

@@ -194,6 +194,7 @@
     'php4': {'text', 'php'},
     'php5': {'text', 'php'},
     'phtml': {'text', 'php'},
+    'piskel': {'text', 'image', 'piskel', 'json'},
     'pl': {'text', 'perl'},
     'plantuml': {'text', 'plantuml'},
     'pm': {'text', 'perl'},