Simplify API by only providing multiple formats

Would normalize the `excerpt` file property to always be `{ html: '...', text: '...' }`

Pro's:
* no need for a `multipleFormats` option (=simpler API)
* could add support for a third format: `stripped`, removing only the outer `

` tags

Cons:
* not backwards-compatible
* can't just put `{{ excerpt }}` in a template

No files, tests, or entry points are named. Start by locating the current excerpt property and multipleFormats option, then trace how templates consume excerpt values. Done means the proposed format and any third format are specified, compatibility implications are resolved, and coverage exists for the resulting API behavior.

Written by the indexing model from the issue text.