Add ability to automatically send yanked text to Clipper

Works on Neovim thanks to the `TextYankPost` autocommand. Can opt out
via `'g:ClipperAuto'`.

This aligns with the set-up I have in recent versions of tmux. Note that
I've explicitly scoped it to explicit yank operations (not deletions or
changes). Given that one tends to yank a lot in Vim, for this to be
usually helpful but with an escape hatch in case the behavior is not
what you want in a specific instance, it's important to pair it with a
tool like Alfred that remembers recent clipboard history, so that you
can go back to a previous entry if you end up inadvertently overwriting
something.

doc/clipper.txt

@@ -25,7 +25,9 @@ runs in the background providing a service that exposes the local clipboard to
 tmux sessions and other processes running both locally and remotely.
 
 Specifically, vim-clipper provides a |:Clip| command, to send the last-yanked
-text to Clipper, and it sets up a <leader>y mapping that calls |:Clip|.
+text to Clipper, it sets up a <leader>y mapping that calls |:Clip|.
+Additionally, if the |TextYankPost| |autocommand| is available, it will send
+the last-yanked text to Clipper automatically.
 
 
 INSTALLATION                                             *clipper-installation*
@@ -82,6 +84,14 @@ OPTIONS                                                       *clipper-options*
   To connect via a UNIX domain socket, set the port number to 0, and provide
   the path to the socket using the |g:ClipperAddress| option.
 
+                                                                *g:ClipperAuto*
+  |g:ClipperAuto|                                          boolean (default: 1)
+
+  Controls whether to use the |TextYankPost| autocommand to route yanked text
+  to Clipper automatically. To prevent this, set to 0 in your |.vimrc|: >
+
+    let g:ClipperAuto=0
+<
                                                                  *g:ClipperMap*
   |g:ClipperMap|                                           boolean (default: 1)
 
@@ -168,6 +178,12 @@ vim-clipper is written and maintained by Greg Hurrell <greg@hurrell.net>.
 
 HISTORY                                                       *clipper-history*
 
+2.0 (25 August 2017)
+
+- Add ability to automatically send yanked text to Clipper using the
+  |TextYankPost| autocommand, and the corresponding |g:ClipperAuto| setting to
+  provide an opt-out mechanism.
+
 1.0 (3 June 2016)
 
 - Rename |<Plug>ClipperClip| to |<Plug>(ClipperClip)|.

plugin/clipper.vim

@@ -22,6 +22,14 @@ if s:map
 endif
 nnoremap <Plug>(ClipperClip) :Clip<CR>
 
+let s:auto=get(g:, 'ClipperAuto', 1)
+if s:auto && exists('##TextYankPost')
+  augroup Clipper
+    autocmd!
+    autocmd TextYankPost * if v:event.operator ==# 'y' | call clipper#private#clip() | endif
+  augroup END
+endif
+
 " Restore 'cpoptions' to its former value.
 let &cpoptions=s:cpoptions
 unlet s:cpoptions