Refactor new help formatting implementation

- Merge formatFileOption() and formatOption() into one.
- Remove unnecessary generality by splitting description formatting into
  its own class, and make OptionsFilter only do option filtering.  All
  output handling is now in OptionsListFormatter (which was renamed from
  OptionsExportFormatter).
- Split formatting basic option name and value placeholders into a
  separate function.

There should be no functional change.  Preparation for a proper synopsis
implementation.

Part of #969.

Change-Id: I900883eb39853ed67d5f6177ae06847283348d9e