add ability to use --bin and --lib together with cargo new and init

[skewballfox]

Problem

currently, cargo init some_project --lib --bin provides the following output:

error: can't specify both lib and binary outputs

Proposed Solution

have a third built in package template that takes a library-first approach

Notes

It seems there is a use-case for projects that are both a library and a binary. clone and compile: its a binary. add to your cargo manifest: it's a library. It also seems there is a right and wrong way to do it.

I think it would be useful to have a package template for library-first applications

[anhaabaete]

For a complete solution, as far better.
Cargo new could have a custom template. From a ~/.cargo/tamplates/my_custom/src/mais.rs
Example:
cargo new --template my_custom

[weihanglo]

For templating, see #5151