[feature] Support submodule of remote theme

[Freed-Wu]

Describe the solution you'd like

A username/reponame may contain a git submodule, like https://github.com/allejo/jekyll-toc.
However, this plugin download https://codeload.github.com/username/reponame/zip/HEAD, which doesn't contain git submodule.

remote_theme:
  url: username/reponame
  submodule: true

Describe alternatives you've considered

remote_theme:
  url: username/reponame
  option: --recurse-submodules