Merge pull request #12 from xeon22/develop

- Added extra settings for plugin in the stash-config.properties file
Remove extraneous ERB whitespace from stash-config.properties
Add empty default['stash']['plugin'] hash attribute
Documentation for Stash plugin attribute/data bag items
Bump to version 2.21.0
Initial chef-stash 3.0.0 changes - see CHANGELOG
Temporarily comment guard-rubocop in Gemfile
Move guard Gems into development group in Gemfile
Add bundler_args to .travis.yml
Comment out all of development group in Gemfile for now
Rubocop auto-corrections
Rubocop corrections and refactoring