Add symbol confirmation

I have this working, I think!

I've tested with prompt_before_searching set to both true and false and got correct results in both cases. I searched for different single symbols that were at the cursor point 5 times and made sure that the search criteria and results were correct. I also changed the symbol via the input panel and verified that the search criteria and results were correct.

As noted at https://github.com/ameyp/CscopeSublime/issues/8, it looks like multiple-symbol search is broken on master and it continues to be broken.

I hope I'm doing this pull request correctly. My goal is to let you review the changes before we merge this to master. =:)

Thanks Amey!