Start `AnswerEntry` component

Renders a button for each pitch class.
On answer button click, set `currentAnswer` state
Add RxJS to `package.json` `dependencies`
Proof of concept: Pass state with RxJS, singleton
State in `main.js`, callback instead of singleton

Been reading a lot about drawbacks of the singleton pattern, so want
to try this alternative.
Rename `currentAnswer` Rx subject to `answers`
Delete unused variable, `rect`
Pull `AnswerEntry` out from `svg` element

Wrap in a `div` to make possible.
`alert` answer entries instead of logging

Just temporarily so the user can see some response to their answer.
Merge branch 'answer-entry-and-state-with-rxjs'
Install `lodash.sample` as a dependency
Teach Staff component to take pitch class as prop

Now it can infer the staff position from the pitch class.

Have `main.js` pass in a random pitch class, pushing that random
selection up from the `Staff` component to its parent. Now `Staff`
just defaults to E rather than randomly selecting.
Show new random note on correct answers
Export pitch classes from Staff component module
Use `Staff.pitchClasses` in `main`
Use `Staff.pitchClasses` in `answer-entry`

Need to clone it with `.slice(0)` before sorting. Otherwise we'll
mutate the underlying array.
Merge branch 'show-new-random-note-on-correct-answers'