Now Docker adopters can use whipper trough Docker Hub's automatically built images.
Thanks to user "anarcat" for proposing and thoroughly explaining how to reach this goal.

Closes #301.

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/

`os.pathconf()` expects a bytes object, but the path is stored as a
unicode object. Since there's no specified encoding, Python chokes when
trying to convert it to a bytes object. Explicitly encoding it as UTF-8
allows Python to change it to a bytes object smoothly.

Fixes #315

As per #318 (comment)

…ncate_filename

Explicitly encode path as UTF-8 in truncate_filename()

Add AppStream metainfo.xml file

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