Feature/add modded aircraft

[red-one1]

added support for modded aircraft
su-15
mig21mf
bronco
yak-28
tu-16
mig-17

[dharmab]

Hi, this deserves a longer response, but I'm currently recovering from a minor but painful injury, so I need to be a bit more brief.

I am planning a different solution to this problem: Allowing users to create their own encyclopedias as JSON or YAML data to override and extend the content shipped inside the binary.

I prefer that approach for multiple reasons:

1. I don't have to update the binary every single time a new mod comes out, just when ED adds a new aircraft to the sim
2. I don't have to gatekeep which mods are "good enough", or handle conflicts between multiple mods
3. Players don't have to redownload the entire application to update the encyclopedia
4. It also allows players to customize other data like threat ranges

For that reason, I am declining to merge this PR, but feel free to maintain your fork until this feature lands in the main branch. Implementation of the feature is fairly trivial using Go's built-in JSON/YAML tags and YAML marshalling, so it'll probably happen soon after I finish #571 and #572.