Add link field for cartridge selling

[avivace]

If the game is sold on cartridge somewhere, a specific link could allow rending a "Buy cartridge and support author" button. E.g. see #139

To be included in Draft 4: #126

[dag7dev]

Wouldn't it be nicer to just leave the link to homepage of the game / project?

I mean, author will take care of updating its homepage with all the links he retain to be more suitable for its needs, I don´t think we really need a specific field to manage buying the cartridge... (note that we already have a "website" field).

We should leave to the author the freedom of managing its own stuff, we shall act like a showcase.