Support for finding symbols

Sometimes I'm not sure if I have used the name of my symbol already. If I use accidentally the same name twice, bad things will happen. Like if I already had a "button" symbol on this document on another page and I just forgot about it, old ones would be replaced.

With this simple approach I can check if there's other symbols with same tag name before syncing.

It's basically a copy of the original "Sync Symbol.sketchplugin" file, but I removed bunch of code and added a simple counter with an alert telling how many symbols are found and on what pages.

It would probably be prettier if we'd combine the common code into a library file and import the library.

(This is a redone pull request as I forgot to branch last time.)