Added plugin to launch external apps

Author: sanchitbahal

iOS/ExternalAppLauncher/README.md

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+# External App Launcher
+
+A Cordova plugin to launch external applications from your Cordova-based iOS application.
+
+### Steps to install
+
+1. Install [cordova-plugman](https://github.com/apache/cordova-plugman).
+2. ```cd``` into your project directory.
+3. Execute the following command:
+
+```
+plugman --platform ios --project . --plugin https://github.com/sbahal/external-app-launcher.git
+```
+
+### Steps to uninstall
+
+```
+plugman --uninstall --platform ios --project . --plugin org.cordova.plugins.ExternalAppLauncher
+```
+
+### Usage
+
+```
+externalApp.launch([appUrlScheme, storeUrl, alertMessage, alertType]);
+```
+where,  
+```appUrlScheme```: the URL scheme of the app that you wish to launch  
+```storeUrl```: the URL to be redirected to in case the external app is not found  
+```alertMessage```: the message to be displayed to the user in case the external app is not found  
+```alertType```: the type of alert to be presented to the user
+  * ```externalApp.alertType.OK```: to just display an alert message, or 
+  * ```externalApp.alertType.OK_CANCEL```: to redirect the user to the specified store  
+
+### Example
+
+```
+externalApp.launch('com.foo.MyApp', '', 'Please download the latest version of this app from the store', externalApp.alertType.OK);
+externalApp.launch('com.foo.MyApp', 'https://foo.com', 'Please download the latest version of this app from the store', externalApp.alertType.OK_CANCEL);
+```