Change init command to create a usable example file

[holmboe]

When running the subgit init command you are rewarded with an empty config file. This doesn't help the user of subgit in any usable way. We should look to vagrant init as a good example on how to do this.

Current behaviour:

$ subgit init
$ cat .subgit.yml
repos: {}

Expected behaviour:

$ subgit init
$ cat .subgit.yml
repos:
  subgit:
    url: git@github.com:dynamist/subgit.git
    revision:
      branch: master

We could even include comments for each key (url, revision, etc).