* feat: extract custom types with extend option
Adds an `extend` option that lets you add custom types to be extracted
and returned alongside the defaults, either in a call to `parse()` or in
a custom extractor.
```
Mercury.parse(
url,
extend: {
last_edited: { selectors: ['#last-edited'], defaultCleaner: false }
}
)
```
* chore: use Reflect.ownKeys
* feat: add CLI options
* doc: add extend param to cli help
* refactor: extract selectExtendedTypes
* feat: only overwrite null extended results
* feat: add allowMultiple extraction option
* feat: accept extendList CLI args
* feat: allow attribute selectors in extends on CLI
* test: update extend tests
* fix: don't invoke cleaner for custom types
* feat: always return array if allowMultiple
* test: add test for array of single result
* refactor: extract extractHtml
* refactor: destructure allowMultiple
* fix: wrap multiple matches in $ for cheerio shim
* fix: find extended types before any other munging
* feat: absolutize all links
* fix: clean content more directly
* doc: Update CLI docs in README
* chore: update dist
* doc: Document extend in custom extractor README
