Fix submodule doesn't update when create release

[numbcoder]

Fix #219

[mattbrictson]

This looks reasonable, but as the sole maintainer of tomo and someone who has never used submodules in a production capacity, I am not comfortable including this in the default tomo git plugin.

As a result, I am going to close this PR.

However, for those interested in using this code in their own tomo projects, you can implement it as follows:

1. Create a plugin local to your project.

# .tomo/plugins/submodule.rb

def update_and_init
  remote.chdir(paths.release) do
    remote.git("--work-tree=. --git-dir=#{paths.git_repo} submodule update --init --recursive --depth=1")
  end
end

2. Load the plugin in your config file

# .tomo/config.rb

plugin "./plugins/submodule"

3. Add the new task right after git:create_release

setup do
  # ...
  run "git:create_release"
  run "submodule:update_and_init"

# ...

deploy do
  # ...
  run "git:create_release"
  run "submodule:update_and_init"

# ...