Cargo templating for `new` and `init`

Cargo templating for `new` and `init`

PR #3004 This is a resubmission of the PR #1747 (from scratch) which adds
support for templating in Cargo. The templates are implemented using the
handlebars crate (where the original PR used mustache).

Examples:
cargo new --template https://url/to/template somedir foo
cargo new --template https://url/to/templates --template-subdir somedir foo
cargo new --template ../path/to/template somedir foo