Internal: Filtering invalidation

- Like sorting, for the new API we need to be able to invalidate data
  held for filtering in a fairly simple manner (it could be done before,
  but it was messy, see fnUpdate - you need to call the methods in the
  parent function, rather than just invalidating). This commit adds that
  ability to DataTables.

- Performance improvements:
  - The big one is that filtering data is only obtained and formatted
    when invalidated, rather than every full filter now.

  - Regular expressions for newline and HTML matching are variables,
    rather than redefined on every call.

  - DIV for reading text version of an HTML formatted string is a
    variable, rather than being recreated on every call.

- Type based formatters have been moved into the extension API ('string'
  and 'html'). They can be overridden there if wanted. Allows
  simplication of the call for the formatter.

- Fixes issue #158 as part of the refactoring.

- Smaller by 22 bytes (compressed)... Likely once invalidation is fully
  implemented that will be swallowed up...