Single Vickers loadout for for N28

[Garven]

Suggestion: Add an option to equip a single vickers on the Nieuport 28.

Reason for Suggestion:  The 94th and 95th only had a single vickers mounted on their N28's at the beginning of their operations at the front.  The 94th Aero squadron only began to equip the 2nd Vickers on June 7 1918 after having been at the front since April 1918.  I feel that this is a relevant detail for career mode that will be coming in the future.