rake: use yard for the api doc
test: set rspec options

* use `--format documentation`
* use `--color`
rake: add `rake test`
social_snippet.rb: use require_relative() instead of require()
spec_helper: require local modules
spec: add tag_parser_spec
spec: add tag_spec
tag_parser: add tag_parser.rb
tag: add tag.rb
tag: add constructor
tag: add Tag.is_tag_line()
test: add tag_test.rb