Fixes #13 Rename using diffs and parse diffs using diff-parse/diff npm modules

[ramya-rao-a]

Changes:

• Update goRename.ts to use gorename -d that would give diffs instead of writing the files on disk with the refactored code
• The formatting tools gofmt, go imports and goreturns have an option to return diffs instead of the whole formatted content. Therefore update goFormat.ts to use the -d option as well.
• When diff tool does not exist on the machine, gorename/gofmt/goimports/goreturns -d will fail. Therefore, in these cases, revert to existing functionality which is write to disk files in case of rename and calculate diffs on our own in case of formatting
• Node modules for generating and parsing diffs
  • diff-match-patch has a way to parse diff content only if the diff is for single file. Since rename can affect multiple files, this module cannot be used by goRename.ts
  • diff module can generate as well as parse diffs. Therefore diff-match-patch has been replaced by diff module

[lukehoban]

This looks good.

A bit of a shame we have to keep both codepaths here, but can't think of a better alternative.

I wasn't able to carefully review all the new diff parsing logic, but in testing in several projects I didn't run into any issues.