Trim trailing whitespace only on modified lines

Fixes: editorconfig#106

plugin/editorconfig.vim

@@ -440,7 +440,8 @@ function! s:ApplyConfig(config) abort " Set the buffer options {{{1
  autocmd! BufWritePre <buffer>
  if s:IsRuleActive('trim_trailing_whitespace', a:config) &&
  \ get(a:config, 'trim_trailing_whitespace', 'false') ==# 'true'
- autocmd BufWritePre <buffer> call s:TrimTrailingWhitespace()
+ autocmd InsertEnter <buffer> let s:pos_on_insert_enter = getpos('.')
+ autocmd InsertLeave <buffer> call s:TrimTrailingWhitespace()
  endif
  augroup END
 
@@ -500,10 +501,11 @@ endfunction
 
 function! s:TrimTrailingWhitespace() " {{{1
  if &l:modifiable
+ let pos = getpos('.')
  " don't lose user position when trimming trailing whitespace
  let s:view = winsaveview()
  try
- silent! keeppatterns keepjumps %s/\s\+$//e
+ execute 'silent! keeppatterns keepjumps :' . s:pos_on_insert_enter[1] . ',' . pos[1] . 's/\s\+$//e'
  finally
  call winrestview(s:view)
  endtry