Use matchadd() instead of :match

Frees up :match for other uses, as seen in #98, also less :exe.

A single matchadd() id should be used because matchadd() goes over from
one buffer to the next, so creating a new id causes
matchadd/matchdelete calls to not work as expected.

It seems that using the same match id across buffers does not seem to
interfere with other open buffers, neither in tabs nor splits.

Author: Cimbali

plugin/better-whitespace.vim

@@ -161,26 +161,25 @@ if g:current_line_whitespace_disabled_soft == 1
 else
     " Match Whitespace on all lines
     function! s:HighlightEOLWhitespace()
+        call <SID>ClearHighlighting()
         if <SID>ShouldHighlight()
-            exe 'match ExtraWhitespace "' . s:eol_whitespace_pattern . '"'
-        else
-            call <SID>ClearHighlighting()
+            let s:match_id = matchadd('ExtraWhitespace', s:eol_whitespace_pattern, 10, get(s:, 'match_id', -1))
         endif
     endfunction
 
     " Match Whitespace on all lines except the current one
     function! s:HighlightEOLWhitespaceExceptCurrentLine()
+        call <SID>ClearHighlighting()
         if <SID>ShouldHighlight()
-            exe 'match ExtraWhitespace "\%<' . line('.') .  'l' . s:eol_whitespace_pattern .
-                                   \ '\|\%>' . line('.') .  'l' . s:eol_whitespace_pattern . '"'
-        else
-            call <SID>ClearHighlighting()
+            let s:match_id = matchadd('ExtraWhitespace',
+                        \   '\%<' . line('.') .  'l' . s:eol_whitespace_pattern .
+                        \ '\|\%>' . line('.') .  'l' . s:eol_whitespace_pattern, 10, get(s:, 'match_id', -1))
         endif
     endfunction
 
     " Remove Whitespace matching
     function! s:ClearHighlighting()
-        match ExtraWhitespace ''
+        silent! call matchdelete(get(s:, 'match_id', -1))
     endfunction
 endif