Add a configurable variable of the highlight of trailing whitespace (#89)

Author: huangzonghao

README.md

@@ -34,6 +34,16 @@ Whitespace highlighting is enabled by default, with a highlight color of red.
     ```vim
     highlight ExtraWhitespace ctermbg=<desired_color>
     ```
+   or
+
+    ```vim
+    let g:better_whitespace_ctermcolor='<desired_color>'
+    ```
+   Similarly, to set gui color:
+
+    ```vim
+    let g:better_whitespace_guicolor='<desired_color>'
+    ```
 
 *  To enable highlighting and stripping whitespace on save by default, use respectively
     ```vim

plugin/better-whitespace.vim

@@ -15,6 +15,10 @@ function! s:InitVariable(var, value)
     endif
 endfunction
 
+" Set the highlight color for trailing whitespaces
+call s:InitVariable('g:better_whitespace_ctermcolor', 'red')
+call s:InitVariable('g:better_whitespace_guicolor', '#FF0000')
+
 " Operator for StripWhitespace (empty to disable)
 call s:InitVariable('g:better_whitespace_operator', '<leader>s')
 
@@ -90,7 +94,7 @@ endfunction
 function! s:WhitespaceInit()
     " Check if the user has already defined highlighting for this group
     if hlexists("ExtraWhitespace") == 0 || synIDattr(synIDtrans(hlID("ExtraWhitespace")), "bg") == -1
-        highlight ExtraWhitespace ctermbg = red guibg = #FF0000
+        execute 'highlight ExtraWhitespace ctermbg = '.g:better_whitespace_ctermcolor. ' guibg = '.g:better_whitespace_guicolor
     endif
     let s:better_whitespace_initialized = 1
 endfunction