[wip] Unify _config.yml into a single template

Use a single `_config.yml` Jinja template for both the `dev` role and the `jenkins_push.yml` playbook, rather than two separate ones. Gotta keep it DRY :umbrella: 

- [x] Parameterize `templates/_config.yml.j2` further to be able to generate the dev version of the config.
- [ ] Provide appropriate variables in `dev_web.yml`.
- [ ] Provide appropriate variables in `jenkins_push.yml`.
- [ ] *(Optional)* provide a way to generate a sane `_config.yml` for non-Vagrant blog writing.
- [ ] Test Vagrant provisioning.
- [ ] Test Jenkins page builds.

Fixes #540.