ENH: use bash associative array for on-the-fly completion (issue #551)

- this reduces the number of functions and allows lazy loading of
  completion options, which makes it easy to quickly add any other
  OpenFOAM application in completion.

  The generic '_of_complete_' function handles (bash) completion for
  any OpenFOAM application. On the first call for any particular
  application, it retrieves the available options from the application
  help output and adds this information to its environmental cache for
  subsequent use.

- Tcsh completion uses the same function via a bash wrapper.
  But since its wrapper is transient, the on-the-fly generation would
  be less efficient. For this case, a pre-generated completion_cache
  can be used, which is generated with

      bin/tools/foamCreateCompletionCache