Code Rewriting

**Core, please hold off on merging this**

I created the interface ICodeRewriter which rewrites a passed code string. The RewritingFilePreProcessor takes a number of rewriters and uses the cumulatively to rewrite passed code.

Created tests for RewritingFilePreProcessor which are essentially the same tests as for FilePreProcessor, but uses StringRewriter, which is a naive sample rewriter.

No additional dependencies created for projects.