Move middleman-templates to cli

Pull request for a discussion.

@bhollis [quite rightly pointed](https://github.com/middleman/middleman/commit/772de85ce332d8c4e50cb486c8671d8688dfd3a2#commitcomment-5841776) out that I had'nt made `middleman-templates` a dependency on `middleman-cli`. A quick solution is to do just that and move the dependency.

When thinking about it I started to wonder if it might be worth gracefully handling when `middleman-templates` is not available. Eg. continue to make `middleman-templates` a `middleman` dependency and when failing to `require 'middleman-templates'` not load the `init` command.

It might not be worth it but just fielding the question. @tdreyno Is the intention of `middleman-cli` to encapsulate all actions or is the decision for abstracting `middleman-cli` to allow for it to be used standalone?