Customize description, icon etc. per-target instead of per-package

[ian-h-chamberlain]

With #60 the groundwork will be laid out to parse TOML for each artifact, to customize icon, description etc. for multiple executables. For now it just uses the package-level metadata, but as mentioned in #44 it would be nice to customize them separately too.

This issue is just broken out so I can do a separate PR. Reposting the example:

[[example]]
name = "ex1"
# I think "description" is non-standard here as it's not documented in
# https://doc.rust-lang.org/cargo/reference/cargo-targets.html#configuring-a-target 
# We could still look for it, but we'd have to manually read in from Cargo.toml 

# examples/ex2.rs may exist but not be defined explicitly in Cargo.toml

[[bin]]
name = "mybin"

# etc. for tests, benches if need be

[package.metadata.cargo-3ds]
romfs_dir = "examples/romfs" # still optional

# should we call these "example", "bin" etc. to match the top-level key?
examples.ex1.romfs_dir = "examples/ex1_romfs"
examples.ex2.romfs_dir = "examples/ex2_romfs"

tests.integration.romfs_dir = "tests/romfs"

# mybin falls back to using "examples/romfs"

# if we needed something else for lib tests we could also do something like this
# but probably it should just use the top level one if we can manage that
lib.romfs_dir = "..."