Add an optional "short title" field to cards

[ngirard]

When dealing with cards, we're dealing with these two potentially conflicting needs:

• providing context to cards and generated reports using titles;
• keeping the maps tight

The exact same problem arises in book editing, where the use of a sole "title" field for chapters cannot always balance between providing enough context and keeping the table of contents and running headers tight enough.

As with book editing, I'd welcome the addition of an optional "short title" field to cards.
The map would display the short titles when they exist, or else the title.
Both title and short title would be exported as CSV.

This would also participate in #30.

[amborle]

While I understand how this could be valuable in certain cases, my initial reaction is that this does not hold enough value for the bulk of users to be added to the product.