Use bin target name instead of package name as default

[rhysd]

Fixes #156

[mdsteele]

Can this just be target.kind.contains("bin")?

[rhysd]

No. It causes a compile error because the iterated item is &String (target.kind is Vec<String>). target.kind.contains(&"bin".to_string()) works, but it causes redundant heap allocation.

error[E0308]: mismatched types
    --> src/bundle/settings.rs:155:57
     |
155  |                     .find(|target| target.kind.contains("bin"))
     |                                                -------- ^^^^^ expected `&String`, found `&str`
     |                                                |
     |                                                arguments to this method are incorrect
     |
     = note: expected reference `&std::string::String`
                found reference `&'static str`
note: method defined here
    --> /Users/rhysd/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/slice/mod.rs:2509:12
     |
2509 |     pub fn contains(&self, x: &T) -> bool
     |            ^^^^^^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `cargo-bundle` (bin "cargo-bundle") due to previous error

[mdsteele]

Which bin does it choose if there is more than one?

[rhysd]

First one is chosen. My understanding is that multiple bin targets are not supported. Shall I explicitly note it here?