Add support for Class.contextType

We already support the static contextTypes and childContextTypes from the legacy
context API. React has now added contextType to the new Context API to make it
easier to consume the context value.

https://reactjs.org/docs/context.html#classcontexttype
https://reactjs.org/blog/2018/10/23/react-v-16-6.html

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## master
+
+- Add support for the static [Class.contextType](https://reactjs.org/docs/context.html#classcontexttype) property
+
 ## 0.9.0-rc3 (2019-11-19)
 
 **[compare](https://github.com/reagent-project/reagent/compare/v0.9.0-rc2...v0.9.0-rc3)**

doc/ReactFeatures.md

@@ -44,6 +44,31 @@ Reagent syntax follows [React Fragment short syntax](https://reactjs.org/docs/fr
           container)
 ```
 
+Alternatively you can use the [static contextType property](https://reactjs.org/docs/context.html#classcontexttype)
+
+```cljs
+(defonce my-context (react/createContext "default"))
+
+(def Provider (.-Provider my-context))
+
+(defn show-context []
+  (r/create-class
+   {:context-type my-context
+    :reagent-render (fn []
+                      [:p (.-context (reagent.core/current-component))])}))
+
+;; Alternatively with metadata on a form-1 component:
+;;
+;; (def show-context
+;;   ^{:context-type my-context}
+;;   (fn []
+;;     [:p (.-context (reagent.core/current-component))]))
+
+(r/render [:> Provider {:value "bar"}
+           [show-context]]
+          container)
+```
+
 Tests contain example of using old React lifecycle Context API (`context-wrapper` function):
 [tests](https://github.com/reagent-project/reagent/blob/master/test/reagenttest/testreagent.cljs#L1141-L1165)
 

src/reagent/impl/component.cljs

@@ -312,7 +312,7 @@
 ;; https://gist.github.com/thheller/7f530b34de1c44589f4e0671e1ef7533#file-es6-class-cljs-L18
 
 (def built-in-static-method-names
-  [:childContextTypes :contextTypes
+  [:childContextTypes :contextTypes :contextType
    :getDerivedStateFromProps :getDerivedStateFromError])
 
 (defn create-class