Add barebones template to conan new

[AbrilRBS]

Changelog: Feature: Add barebones template option to conan new.
Docs: conan-io/docs#2881

While testing some functionality, I had the need to create a very basic recipe to test some commands on. As I thought that "there probably is a template for it in conan new" and was disappointed when it wasn't.

I had to implement the ability to add multiple values to the same definition, so something like -d requires=math/1.0 -d requires=ai/1.3 would result in

def requirements(self):
  self.requires("math/1.0")
  self.requires("ai/1.3")

without breaking

• I've followed the PEP8 style guides for Python code.
• (Will do) I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[memsharded]

I like the idea of having some other convenient templates, they are useful for teaching, testing, examples, etc. I think we can also provide other improvements for the templates, like some templates with requires (not in this PR, just an idea).

Please check the comments, add some simple test to cover it, and it should be good.

[AbrilRBS]

Thanks for the reviews, I've implemented most of the requested changes, with only the explanation comments left to do

[memsharded]

Looking good