Support deleting folder content or cancel convert

Why:

 * Currently when generating we just add files to folder causing
   duplicates. Better if we could keep the folder clean or overwrite files
   instead of creating duplicates.

This change addreses the need by:

 * Offer to delete the folder if already found or cancel if you do not want
   to delete.

converttoasciidoc.gapps

@@ -1,3 +1,5 @@
+var ADOC_MIMETYPE = "text/asciidoc";
+
 // Open handler to add Menu
 function onOpen(e) {
   var ui = DocumentApp.getUi();
@@ -50,7 +52,7 @@ function ConvertToAsciiDocEmail() {
 
   // Add asciidoc document to attachments
   convertedDoc.attachments.push({"fileName":DocumentApp.getActiveDocument().getName()+".adoc", 
-                                 "mimeType": "text/plain", "content": convertedDoc.text});
+                                 "mimeType": ADOC_MIMETYPE, "content": convertedDoc.text});
 
   // In some cases user email is not accessible  
   var mail = Session.getActiveUser().getEmail(); 
@@ -107,7 +109,21 @@ function ConvertToAsciiDocFile() {
     found =  parent.createFolder(targetDirName);
   } else {
     Logger.log("Reusing existing target");
-  } // TODO: inform user if we create or overwrite ?
+    var ui = DocumentApp.getUi();
+    var result = ui.alert('Existing target folder found!', 'You already have "' + targetDirName + '" in your Drive. Delete folder or Cancel ?', 
+                          ui.ButtonSet.OK_CANCEL)
+
+    if(result == ui.Button.OK) {
+        Logger.log("Trashing target folder..."); 
+        found.setTrashed(true); 
+        Logger.log("Re-Creating output folder..."); 
+        found =  parent.createFolder(targetDirName);
+
+    } else {
+        Logger.log("Do not delete target folder, stopping!"); 
+        return;
+    }
+  }
 
   // Write all files to target folder
   for(var file in convertedDoc.files) {
@@ -121,7 +137,7 @@ function ConvertToAsciiDocFile() {
   }
 
   // Write mardown file to target folder
-  found.createFile(DocumentApp.getActiveDocument().getName() + ".adoc", convertedDoc.text, "text/plain");   
+  found.createFile(DocumentApp.getActiveDocument().getName() + ".adoc", convertedDoc.text, ADOC_MIMETYPE);   
 }
 
 function processSection(section) {