feat: Support multiple templates in a single git repostory

[WolverineFan]

Is your feature request related to a problem? Please describe.
I would love to be able to put multiple templates in a single git repository. Currently, when I reference a git URL to run Copier it only seems to find a copier.yml file in the root directory of the repo.

Describe the solution you'd like
Ideally I would like to be able to reference a path to the template root from the command-line. Given a repo structure like this:

.
├── .git
├── template_1
│   ├── copier.yml
│   ├── file.txt
│   └── template.txt.jinja
├── template_2
│   ├── copier.yml
│   ├── dynamic.txt.jinja
│   └── static.txt
└── template_3
    ├── copier.yml
    ├── sample.yaml
    └── template.yaml.jinja

I can think of two common patterns that could work:
copier --vcs-ref HEAD git@github.com:DeloitteHux/hux-k8s-cd-cdm-template.git//template_1 /path/to/render
copier --vcs-ref HEAD --template-root template_1 git@github.com:DeloitteHux/hux-k8s-cd-cdm-template.git /path/to/render

Describe alternatives you've considered
Multiple repositories, which is fine to a degree, but there are use-cases where we want a single repo with multiple templates.

[yajo]

This is answered in the docs: https://copier.readthedocs.io/en/latest/configuring/#subdirectory

Duplicate of #315