Add topspace-active, improve topspace-autocenter-buffers

[trevorpogue]

Resolves #3.

• Add customizable variable topspace-active to determine when topspace-mode mode is active / has any effect on buffer.
  It can be set to t, nil, or a predicate function. This is useful in particular when global-topspace-mode is enabled but you want
  topspace-mode to be inactive in certain buffers or in any specific
  circumstance. When inactive, topspace-mode will still technically be on,
  but will be effectively off and have no effect on the buffer.

• Allow topspace-autocenter-buffers to be either t, nil, or a predicate function (before it could only be t or nil)

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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 or M-x checkdoc warnings -- False positives for text with topspace-mode
• I've updated the changelog (if adding/changing user-visible functionality) -- We're going to transition to automatic changelog generator instead
• I've updated the readme (if adding/changing user-visible functionality)
• I have confirmed some of these without doing them