prevent windows from closing on middle_click

Adding an option to prevent windows from being closed when a buffer in
the tabline is middle clicked and the clicked buffer is currently open
in a window.

When this option is enabled, instead of closing the window a new buffer
will be opened in all of the windows editing the clicked buffer instead.

This is my first pull request AND my first experience with vimscript, so
my apologies if this is a bit sloppy 😄

doc/airline.txt

@@ -145,7 +145,6 @@ values):
  >
   let g:airline_skip_empty_sections = 1
 <
-
 ==============================================================================
 COMMANDS                                                  *airline-commands*
 
@@ -693,6 +692,10 @@ Note: Enabling this extension will modify 'showtabline' and 'guioptions'.
    won't update airline on |:badd| commands) >
   let airline#extensions#tabline#disable_refresh = 0
 
+* preserve windows when closing a buffer from the bufferline (default: 0) >
+
+  let airline#extensions#tabline#middle_click_preserves_windows = 1
+<
 -------------------------------------                     *airline-tmuxline*
 tmuxline <https://github.com/edkolev/tmuxline.vim>