support custom boxen clone URL templates

surfaces `config.repotmpl`, which serves as a clone URL template for boxen. Set via `BOXEN_REPO_URL_TEMPLATE` environment variable and defaults to `https://github.com/%s`, where %s should be replaced with the repository (in user/repo format). generalization of boxen/boxen#58.

/cc @jbarnette @bhuga @mdelagralfo