Enable uppercase, lowercase, titlecase shortcuts.

Addresses #3469.

* Add titleize utility function.
  * Capitalizes first word.
  * Capitalizes all words not contained in simple article/conjunction
    list.
* Enable shortcuts for `uppercase`, `lowercase`, and `titlecase`.
* Fix header shortcuts
  * Ensure that header shortcuts do not duplicate text.
  * Make headers idempotent (pressing `ctrl+alt+1` then `ctrl+alt+2` does not make `# # # blah`.

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Will somewhat conflict with the changes in #3471 (will just need minor copy/pasta changes) so one or the other will need to be rebased after the first is merged.