chore(release): 2.18.0

# [2.18.0](v2.17.1...v2.18.0) (2020-03-11)

### Bug Fixes

* validate `input[list]` ([9c70db24](9c70db24))
* **rules:** `no-dup-id` handles when id is set but omitted value ([5f678a56](5f678a56))
* **rules:** `no-missing-references` ignores omitted references ([b8863cd8](b8863cd8))
* **rules:** add contextual documentation for `deprecated` ([7fbf433f](7fbf433f))
* **rules:** add contextual documentation for `element-name` ([2a98bada](2a98bada))
* **rules:** better and more contextual messages for `deprecated` ([3602be73](3602be73))
* **rules:** contextual documentation for `deprecated-rule` ([8b106014](8b106014))
* **rules:** improve documentation for `doctype-html` ([1a896a87](1a896a87))
* **shim:** expose `ConfigError` and `UserError` ([2d002c79](2d002c79))
* **transform:** ignore non-string values in `TemplateExtractor` ([7f27c8b6](7f27c8b6))
* allow both null and empty string when attribute allows empty values ([5b6991b6](5b6991b6))

### Features

* `attribute-allowed-values` handle omitted values ([962d0791](962d0791))
* new rule `attribute-empty-style` ([a328b558](a328b558))