Add quotes to command arguments

I was having fun with this gem but found a little bug as described below:

When I tried to call:

```ruby
TerminalNotifier.notify("foo", title: "[bar] foobar")
```

The title wouldn't be shown properly but the default title would be shown instead, and it appears that it's the open bracket that causes the problem.

I just simply added quotes to command arguments, and it was fixed now.