Support book tracking.

Support book tracking apps such as https://indiebookclub.biz.

Example error when trying to post right now.
```
2020-12-13T15:54:39.137449034Z app[web.1]: {
2020-12-13T15:54:39.137524787Z app[web.1]: type: [ 'h-entry' ],
2020-12-13T15:54:39.137534194Z app[web.1]: properties: {
2020-12-13T15:54:39.137540870Z app[web.1]: summary: [
2020-12-13T15:54:39.137547424Z app[web.1]: 'Want to read: Gödel, Escher, Bach: An Eternal Golden Braid by Douglas R. Hofstadter, ISBN: 9780465026562'
2020-12-13T15:54:39.137555859Z app[web.1]: ],
2020-12-13T15:54:39.137562226Z app[web.1]: 'read-status': [ 'to-read' ],
2020-12-13T15:54:39.137585080Z app[web.1]: 'read-of': [ [Object] ],
2020-12-13T15:54:39.137592721Z app[web.1]: category: [ 'reccomended-to-me' ]
2020-12-13T15:54:39.137599434Z app[web.1]: }
2020-12-13T15:54:39.137605734Z app[web.1]: }
2020-12-13T15:54:39.157787856Z app[web.1]: (node:25) UnhandledPromiseRejectionWarning: TypeError: Cannot read property '0' of undefined
2020-12-13T15:54:39.157854456Z app[web.1]: at file:///usr/src/app/src/micropub.ts:108:39
2020-12-13T15:54:39.157864038Z app[web.1]: at processTicksAndRejections (internal/process/task_queues.js:97:5)
2020-12-13T15:54:39.158236516Z app[web.1]: (node:25) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
2020-12-13T15:54:39.158569152Z app[web.1]: (node:25) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```

Start in src/micropub.ts at line 108 and reproduce the failure with the h-entry payload shown in the issue, including read-status and read-of. Trace which property lookup produces the undefined value, then verify that a book-tracking post such as the example is accepted without the TypeError.

Written by the indexing model from the issue text.