Add "then by" sort parameter binding. #14

[rapito]

Added a thenBy binding to support multi sorting after the first sort field.

• Currently uses the same order direction as the first field.
• This was also mentioned on issue #14

Changes

• Refactor sortand init method to handle new attribute.
• Add sortThenBy method to handle sorting results of thenBy fields
• Update Readme.md usages section.
• Add Example html.