Change assets to produce sub-folders

[chotchki]

I ran into an issue in developing a bevy game that loads assets from an asset folder which I've further organized into nested subfolders. At the moment the project.yml generated doesn't preserve this folder structure.

I've changed this behavior to instead create a subfolder per asset listed in Cargo.toml. This is a breaking change to how the configuration syntax works and I am open to resubmitting this pull request if another approach is desired.

[Gordon-F]

@chotchki Hi! Thank you for PR! What if you change assets folder to you subfolder in Cargo.toml manifest without changing source code? Is it help in your use case?

[package.metadata.ios]
# Specifies an assets folder.
assets = ["assets/subfolder"]