Improve zsh completion

Add test name completion from Cargo.toml

Add package name completion from Cargo.toml

Rename --name to --<target-name>
auto merge of #881 : eerden/cargo/zsh-completion-improvements, r=alexcrichton

Some small improvements:

Add test name completion from Cargo.toml

Add package name completion from Cargo.toml

Rename `--name` to `--\<target-name\>`