Add spec for error on omitted `done` for `parse`

Additionally, includes spec for `applyPlugins`.

Issue https://github.com/wooorm/retext/issues/18.
Add interface for error on omitted `done` for `parse`

Additionally, includes interface for `applyPlugins`.

Issue https://github.com/wooorm/retext/issues/18.
Merge branch 'feature/fail-without-callback'

Add error on omitted `done` for `parse` and `applyPlugins`.

This is related to 27dbfd7, `Use ware to support async parsing`.
Since which the interface changed, making previous code invalid.
Throwing an error helps users to detect these changes early.

Closes https://github.com/wooorm/retext/issues/18.