Turn --git on by default for `cargo-new`.

This adds a command-line --no-git option to disable this behavior, as well as
adding a global config section for `git = false`. While I was at it I write some
documentation for the configuration format that cargo uses.
auto merge of #470 : alexcrichton/cargo/cargo-new-git, r=brson

This adds a command-line --no-git option to disable this behavior, as well as
adding a global config section for `git = false`. While I was at it I write some
documentation for the configuration format that cargo uses.