Allow commenting empty lines given configuration (#250)

* Allow commenting empty lines given configuration

    If g:NERDCommentEmptyLines=1 then it will comment empty line, which is
    useful when commenting blocks.

    When uncommenting it will delete any trailing whitespace.

* Add documentation for NERDCommentEmptyLines

README.md

@@ -88,6 +88,9 @@ let g:NERDAltDelims_java = 1
 
 " Add your own custom formats or override the defaults
 let g:NERDCustomDelimiters = { 'c': { 'left': '/**','right': '*/' } }
+
+" Allow commenting and inverting empty lines (useful when commenting a region)
+g:NERDCommentEmptyLines = 1
 ```
 
 ### Default mappings