Fix yard warnings.
Add feature: tag parsing and filtering.

* Add EntryContainer#filter that accepts a tag name and value.
  Returns a flat-map of all children with matching tags
* Entry:
    - #text_with_trailing_tags: insert colon between text and trailing tags for projects.
    - introduce #tag? to check for existence of tag with or without specifice value
    - rename #tag to #tag_value and have it return the value, '' or nil
Change all incorrect uses of rspec `eql` to `eq`.