* 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`.
