Fix bug where topspace-mode doesn't work locally

[trevorpogue]

Resolves #5

• Allow topspace to work locally
• Add guards around advice-add functions to only modify functions when topspace is both enabled and active in current buffer
• Never remove advice-add because the modified functions will effectively not be changed when topspace is not enabled. Removing the advice-add at the correct time may be a bit difficult and error-prone because this would mean keeping track of when topspace is no longer enabled in any buffers (but not impossible to do maybe later on)
• A few minor cosmetic changes and clarifications in docstrings

---

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