Add script to refresh git repos

refresh.rb goes through each data file and populates a list of git
repositories. It then clones the repos or does fetches HEAD if they've
already been cloned.

The purpose of this script is to serve up-to-date themes and plugins
with sachet.
Refactor refresh script and add 'repos' to gitignore

Clean-up refresh script. The script looks good for now and will be later
revisited if we need to modify it to handle additional cases.

The directory 'repos' has been added to .gitignore to avoid pushing
cloned repos to the sachet repository.