Add topspace-empty-line-indicator defcustom

[trevorpogue]

Resolves #7

• By default, "~" will be displayed on each top space line as an empty-line indicator
  if indicate-empty-lines is non-nil, and the indicator string's 'face is set to 'fringe
• The empty-line indicator string is customizable through topspace-empty-line-indicator
• The string properties and conditions in which it is present are also customizable through
  topspace-empty-line-indicator by setting it to a function instead of a constant string.
  Whenever you don't want any indicator present, have the function return "" (an empty string).

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Checklist

• I have read the topspace contributing guidelines
• My changes follow the Emacs Lisp conventions and the Emacs Lisp Style Guide
• I've used the latest version of package-lint to check for packaging issues, and addressed its feedback
• The new code is not generating bytecode warnings
• I've updated the readme (if adding/changing user-visible functionality)
• I have confirmed some of these without doing them