Handle covert leaves differently

[lynn]

• Get rid of leaf.word === 'functional'
  • It was mostly just a hack added to not draw ∅ under a 𝘷.
• If leaf.word === 'covert', then leaf.value says what the "covert value" is.
  • Right now the possibilities are ∅ (a generic null), BE (non-agentive 𝘷), CAUSE (agentive 𝘷), and PRO.
• Get rid of the hacky 𝘷0 label, in favor of {label: '𝘷', word: 'covert', value: 'BE'}.
• "Plain" 𝘷 is now {label: '𝘷', word: 'covert', value: 'CAUSE'}.

[robintown]

Looks good with one suggestion (and go ahead with the covert: boolean approach if you want)