move "if new_tag" logic from Repeater#scan and put it in Token#tag.

small refactoring. Moved the if then logic from Repeater#scan into Token#tag to follow a better tell don't ask pattern.  0 failing tests.

```ruby 
    def self.scan(tokens, options)
      tokens.each do |token|
        if t = scan_for_season_names(token, options) then token.tag(t); next end
        if t = scan_for_month_names(token, options) then token.tag(t); next end
        if t = scan_for_day_names(token, options) then token.tag(t); next end
        if t = scan_for_day_portions(token, options) then token.tag(t); next end
        if t = scan_for_times(token, options) then token.tag(t); next end
        if t = scan_for_units(token, options) then token.tag(t); next end
      end
    end
```
to 
```ruby 
    def self.scan(tokens, options)
      tokens.each do |token|
        token.tag scan_for_season_names(token, options)
        token.tag scan_for_month_names(token, options)
        token.tag scan_for_day_names(token, options)
        token.tag scan_for_day_portions(token, options)
        token.tag scan_for_times(token, options)
        token.tag scan_for_units(token, options)
      end
    end
```


and simply added if new_tag
```ruby
    def tag(new_tag)
      @tags << new_tag if new_tag
    end
```