Fixing issue with double dollars in replace string

I picked this idea from https://github.com/adobe/brackets/pull/5772#issuecomment-27436833.
This should be exactly the same dollar-signs-in-replace-with-string behaviour as JavaScript itself haves.

I did a bit of testing and everything worked fine (for example `$1`, `$$1`, `$$$1`, `$`, `$$`).

FindReplace tests passing, too.