Fix mozilla/thimble.mozilla.org#1564 - Check for existing file with name before creating a new file

[gideonthomas]

To test this (try on both master and this pr):

• npm start
• Open your console and type in bramble.addNewFile({basenamePrefix: "script", contents: "Hi", ext: ".js"}, function(err) { console.log("Added new file with: ", err); });. This should create a script-1.js file.
• Refresh the page
• Run that command in the console again

In my patch, after you run the second command you should have a script-2.js file; on master, it should overwrite your script-1.js file.

@humphd r?

[humphd]

Just so it doesn't read like a mistake, let's wrap _nextUntitledIndexToUse++ in (...)

[humphd]

This works great! Just make that one fix to the code I mentioned above and r=me.