Preserve original indentation settings if IndentFinder cannot draw a conclusion.

In the original code, the final indentation would be set to DEFAULT_RESULT when the indentation cannot be determined. I think it would be better if it leaves the vimrc settings untouched; if the result is 'tab', the 'tabstop' option should also be preserved. I have modified the code to make it do so. By the way, I added a vim help file and README.md, content of which was excerpted from the original IndentFinder homepage (http://www.freehackers.org/Indent_Finder).