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

doc/NERD_commenter.txt

@@ -437,6 +437,9 @@ change the filetype back: >
 |'NERDBlockComIgnoreEmpty'|           Forces right delimiters to be placed
                                       when doing visual-block comments.
 
+|'NERDCommentEmptyLines'|             Specifies if empty lines should be
+                                      commented (useful with regions).
+
 |'NERDCommentWholeLinesInVMode'|      Changes behaviour of visual comments.
 
 |'NERDCreateDefaultMappings'|         Turn the default mappings on/off.
@@ -559,6 +562,15 @@ Otherwise, the code block would become: >
     }
     /*}  */
 <
+------------------------------------------------------------------------------
+                                                     *'NERDCommentEmptyLines'*
+Values: 0 or 1.
+Default: 0.
+
+This option affects commenting of empty lines. If this option is turned on,
+then empty lines will be commented as well. Useful when commenting regions of
+code.
+
 ------------------------------------------------------------------------------
                                                 *'NERDCommentWholeLinesInVMode'*
 Values: 0, 1 or 2.

plugin/NERD_commenter.vim

@@ -1810,8 +1810,9 @@ function s:CanToggleCommentLine(forceNested, lineNum)
         return 0
     endif
 
-    " make sure we don't comment lines that are just spaces or tabs or empty.
+    " make sure we don't comment lines that are just spaces or tabs or empty,
-    if theLine =~ "^[ \t]*$"
+    " unless configured otherwise
+    if g:NERDCommentEmptyLines == 0 && theLine =~ "^[ \t]*$"
         return 0
     endif