Add speakers input to CLI

[BenDMyers]

Sometimes events (especially talks!) will have a speakers field in their frontmatter, which is an array of speaker names. However, the command line tool (npm run gen or yarn gen) doesn't provide any means of supplying speakers for a given event. There's also not a great discovery mechanism to help people find out that you can supply speakers.

The CLI should provide a means of optionally supplying a list of speakers for a new event.

• If the CLI user chooses not to supply speakers, the generated event should not have a speakers property in its frontmatter.
• If the CLI user supplies speakers, they should be able to supply one to many speakers.
• The speakers field should be formatted as YAML's bulleted list — in other words, like:

speakers:
  - Grace Hopper
  - Ada Lovelace