Add filetype blacklist

Exist languages with significant trailing whitespace, e.g. Markdown.
Adds the flag `b:strip_trailing_whitespace_enabled` for selectively
disabling the plugin on a per-buffer basis. The tracking of modified
lines still always has to happen though, to support re-enabling the
plugin.

In the autocmd group `strip_trailing_whitespace_filetype` an autocommand
is added for the `FileType` event that blacklists diff and Markdown
files and activates the plugin for other filetypes. To get rid of the
default behaviour:

    :autocmd! strip_trailing_whitespace_filetype

Alternatively an explicit global variable could have been added as a
more easily customizable blacklist. However, since a buffer toggle is
warranted also for one-off situations, this was not preferred.

Author: axelf4

plugin/strip_trailing_whitespace.vim

@@ -218,6 +218,8 @@ function s:StripTree(n, offset) abort
 endfunction
 
 function s:OnWrite() abort
+	if !get(b:, 'strip_trailing_whitespace_enabled', 1) | return | endif
+
 	if !has('nvim') | call listener_flush() | endif
 
 	let s:is_stripping = 1
@@ -237,4 +239,10 @@ augroup strip_trailing_whitespace
 	autocmd BufWritePre * call s:OnWrite()
 augroup END
 
+augroup strip_trailing_whitespace_filetype
+	autocmd!
+	autocmd FileType * let b:strip_trailing_whitespace_enabled = index(['diff', 'markdown'],
+				\ &filetype) == -1
+augroup END
+
 let &cpo = s:save_cpo | unlet s:save_cpo

test/test.vim

@@ -26,3 +26,28 @@ function Test_AddLineAboveChange() abort
 	endfunction
 	call s:TestEdits(['foo '], function('s:EditCb'), ['', 'zoo'])
 endfunction
+
+function Test_AbleToDisable() abort
+	function! s:EditCb() abort
+		let b:strip_trailing_whitespace_enabled = 0
+		normal! rf
+	endfunction
+	call s:TestEdits(['line '], function('s:EditCb'), ['fine '])
+
+	" Should remember modification sites even if disabled
+	function! s:EditCb() abort
+		let b:strip_trailing_whitespace_enabled = 0
+		normal! rf
+		silent write
+		let b:strip_trailing_whitespace_enabled = 1
+	endfunction
+	call s:TestEdits(['line '], function('s:EditCb'), ['fine'])
+endfunction
+
+function Test_DisabledForMarkdown() abort
+	function! s:EditCb() abort
+		set filetype=markdown
+		normal! rf
+	endfunction
+	call s:TestEdits(['line '], function('s:EditCb'), ['fine '])
+endfunction