allow an option to specify to download with SSH or HTTPs from Git

Ref. [lib/dotstrap/git.rb:10](https://github.com/mkwmms/dotstrap/blob/028fa44ef578e3fbbd9ec9b1a3910d4b2539bdb3/lib/dotstrap/git.rb#L10)

``` rb
      # TODO: allow an option to specify to download with SSH or HTTPs from Git
      @url = "https://github.com/#{@repo}"
      partition = @repo.partition("/")
      @github_user = partition[0]
…
```
