def initialize(worksheet, options={})
      DataTypeValidator.validate "Hyperlink.worksheet", [Worksheet], worksheet
      @worksheet = worksheet
      @target = :external
      parse_options options
      yield self if block_given?
    end
Creates a new hyperlink object.
@note the preferred way to add hyperlinks to your worksheet is the Worksheet#add_hyperlink method
@param [Worksheet] worksheet the Worksheet that owns this hyperlink
@param [Hash] options options to use when creating this hyperlink
@option [String] display Display string, if different from string in string table. This is a property on the hyperlink object, but does not need to appear in the spreadsheet application UI.
@option [String] location Location within target. If target is a workbook (or this workbook) this shall refer to a sheet and cell or a defined name. Can also be an HTML anchor if target is HTML file.
@option [String] tooltip The tip to display when the user positions the mouse cursor over this hyperlink
@option [Symbol] target This is :external by default. If you set it to anything else, the location is interpreted to be the current workbook.
@option [String|Cell] ref The location of this hyperlink in the worksheet