Allow test source files to declare custom vim options

The syntax is:

    # extra-option: let whatever=1
Add source files for GH #166
Add a helper script to launch a pristine Vim
Fix highlighting of subroutine signatures
Test folds for subroutine signatures
Update docs on defaults for Perl settings
Merge commit 'a7b96b02cbc012fb000de385279ed9f10dd99c06'

* commit 'a7b96b02cbc012fb000de385279ed9f10dd99c06':
  Update docs on defaults for Perl settings
  Test folds for subroutine signatures
  Fix highlighting of subroutine signatures
  Add a helper script to launch a pristine Vim
  Add source files for GH #166
  Allow test source files to declare custom vim options