Add initial AppStream metainfo.xml file

AppStream metainfo.xml files are used to provide "metadata necessary for
an application-centric view on package repositories." This is an initial
and somewhat basic one written from scratch for whipper.

See https://www.freedesktop.org/wiki/Distributions/AppStream/
and https://www.freedesktop.org/software/appstream/docs/

setup.py

@@ -14,5 +14,8 @@ setup(
         'console_scripts': [
             'whipper = whipper.command.main:main'
          ]
-    }
+    },
+    data_files=[
+        ('share/metainfo', ['com.github.whipper_team.Whipper.metainfo.xml']),
+    ],
 )