Setup email linking settings option [perrybutler#52]

wp-oauth-settings.php

@@ -473,6 +473,14 @@ function wpoa_cc_ux() {
 					<p class="tip-message">Prevents WordPress from sending an email to newly registered users by default, which contains their username and password.</p>
 				</td>
 				</tr>
+
+				<tr valign='top' class="has-tip">
+				<th scope='row'>Allow email linking when loggin in: <a href="#" class="tip-button">[?]</a></th>
+				<td>
+					<input type='checkbox' name='wpoa_email_linking' value='1' <?php checked(get_option('wpoa_email_linking') == 1); ?> />
+					<p class="tip-message">Increases permission request from user to include email in order to match authenticated user with registered users. Because this exposes more user data, please use at your discretion.</p>
+				</td>
+				</tr>
 
 				<tr valign='top' class="has-tip">
 				<th scope='row'>Assign new users to the following role: <a href="#" class="tip-button">[?]</a></th>

wp-oauth.php

@@ -80,6 +80,7 @@ public static function get_instance() {
 				),
 			),
 		'wpoa_suppress_welcome_email' => 0,								// 0, 1
+		'wpoa_email_linking' => 0,										// 0, 1
 		'wpoa_new_user_role' => 'contributor',							// role
 		'wpoa_google_api_enabled' => 0,									// 0, 1
 		'wpoa_google_api_id' => '',										// any string