jquery.tablesorter: Minor coding style cleanup and DRY

jquery.tablesorter: Minor coding style cleanup and DRY

* Consistently name 'length' cached variables 'len' (e.g. not 'l').

* Fix invalid documentation syntax for convertSortList().
  (Build is passing because this file is not yet indexed.)

* Remove redundant 'len' variable from pure JavaScript arrays where
  iterations can't change the length. Caching the length is a dated
  practice meant to micro-optimise execution time. For the moment,
  kept any loops with complex iteration bodies as-is, as well as loops
  over structures that are not pure arrays (e.g. NodeList objects).

* Use getParserById() in addParser() instead of duplicating its logic.

Change-Id: I566eff6efd97b5d9672277bacb4cb2b501f4625f