Allow to configure the templates folder

[nlohmann]

Right now, the templates folder is hard coded, for instance:

reuse-tool/src/reuse/_annotate.py

Line 49 in a7896a5

template_dir = project.root / ".reuse/templates"

This forces projects that want to use reuse annotate to have a .reuse/templates folder in their repository which can be a bit annoying. It would be awesome if the templates folder can be configured with a command-line argument or an environment variable such that one can move it to a more canonical place like extra/reuse/templates or so.