"subject" field for Share

Summary:
Add field `subject` to `Options` when Share.
Check 9ee815f6b5/Libraries/ActionSheetIOS/ActionSheetIOS.js (L52)

iOS:
The field `subject` is obtained from `Options`
1e8f3b1102/Libraries/ActionSheetIOS/RCTActionSheetManager.m (L159)

**iOS:**
Tested the following components `ActionSheetIOS` and `Share` using RNTester

**Android**
Don't use the key `subject`
Check 1e8f3b1102/ReactAndroid/src/main/java/com/facebook/react/modules/share/ShareModule.java (L52)
Closes https://github.com/facebook/react-native/pull/15800

Differential Revision: D6060998

Pulled By: hramos

fbshipit-source-id: fd06f680a4705989e397c5700d8ab35aa7129ba6

Libraries/Share/Share.js

@@ -22,7 +22,12 @@ const {
 } = require('NativeModules');
 
 type Content = { title?: string, message: string } | { title?: string, url: string };
-type Options = { dialogTitle?: string, excludedActivityTypes?: Array<string>, tintColor?: string };
+type Options = {
+  dialogTitle?: string,
+  excludedActivityTypes?: Array<string>,
+  tintColor?: string,
+  subject?: string
+};
 
 class Share {
 
@@ -50,12 +55,13 @@ class Share {
    *
    * #### iOS
    *
-   * - `excludedActivityTypes`
-   * - `tintColor`
+   *  - `subject` - a subject to share via email
+   *  - `excludedActivityTypes`
+   *  - `tintColor`
    *
    * #### Android
    *
-   * - `dialogTitle`
+   *  - `dialogTitle`
    *
    */
   static share(content: Content, options: Options = {}): Promise<Object> {

RNTester/js/ShareExample.js

@@ -82,6 +82,7 @@ class ShareMessageExample extends React.Component<$FlowFixMeProps, any> {
       url: 'http://facebook.github.io/react-native/',
       title: 'React Native'
     }, {
+      subject: 'A subject to go in the email heading',
       dialogTitle: 'Share React Native website',
       excludedActivityTypes: [
         'com.apple.UIKit.activity.PostToTwitter'