Allow users to set default config variables

When using the CLI to generate a new theme, it'd be great if users could set a couple of default variables, maybe in a config file somewhere in the root folder, or by passing a global argument the same way you can with `git config`.

Specifically for: author name, author URL, and author email, which I think in most users' cases would be consistent for all projects. If I could set these variables once and forget about them, that would be pretty great.

Side note: what is author email used for? I can't find the email address used anywhere in the generated files, and I wonder if some users might be leery about giving information like that, particuarly when it doesn't seem to be relevant to the end result.