Issue #128: Make saving of GUI font optional

Issue 128 on GitHub:
  #128

README.md

@@ -183,6 +183,12 @@ The session load/save prompts are quite verbose by default because they explain
 
 By default this option is set to false (0). When you set this option to true (1) and you start Vim, the session plug-in will open your last used session instead of the default session. Note that the session plug-in will still show you the dialog asking whether you want to restore the last used session. To get rid of the dialog you have to set `g:session_autoload` to `'yes'`.
 
+### The `g:session_persist_font` option
+
+By default the plug-in will save the GUI font with the session to be reused the next time that session is loaded, this can be disabled by adding the following line to your [vimrc script] [vimrc]:
+
+    :let g:session_persist_font = 0
+
 ### The `g:session_persist_colors` option
 
 By default the plug-in will save the color scheme and the ['background' option] [bg] with the session to be reused the next time that session is loaded, this can be disabled by adding the following line to your [vimrc script] [vimrc]:

autoload/xolox/session.vim

@@ -1,10 +1,10 @@
 " Public API for the vim-session plug-in.
 "
 " Author: Peter Odding
-" Last Change: July 8, 2015
+" Last Change: September 4, 2015
 " URL: http://peterodding.com/code/vim/session/
 
-let g:xolox#session#version = '2.12'
+let g:xolox#session#version = '2.13'
 
 " Public API for session persistence. {{{1
 
@@ -40,7 +40,9 @@ function! xolox#session#save_session(commands, filename) " {{{2
   " the session is restored in a GUI Vim, things will look funky :-).
   if has('gui') && is_all_tabs
     call add(a:commands, 'set guioptions=' . escape(&go, ' "\'))
-    call add(a:commands, 'silent! set guifont=' . escape(&gfn, ' "\'))
+    if xolox#misc#option#get('session_persist_font', 1)
+      call add(a:commands, 'silent! set guifont=' . escape(&gfn, ' "\'))
+    endif
   endif
   call xolox#session#save_globals(a:commands)
   if is_all_tabs

doc/session.txt

@@ -32,13 +32,14 @@ Contents ~
   12. The |g:session_autosave_silent| option
   13. The |g:session_verbose_messages| option
   14. The |g:session_default_to_last| option
-  15. The |g:session_persist_colors| option
-  16. The |g:session_persist_globals| option
-  17. The |g:session_restart_environment| option
-  18. The |g:session_command_aliases| option
-  19. The |g:session_menu| option
-  20. The |g:session_name_suggestion_function| option
-  21. The |g:loaded_session| option
+  15. The |g:session_persist_font| option
+  16. The |g:session_persist_colors| option
+  17. The |g:session_persist_globals| option
+  18. The |g:session_restart_environment| option
+  19. The |g:session_command_aliases| option
+  20. The |g:session_menu| option
+  21. The |g:session_name_suggestion_function| option
+  22. The |g:loaded_session| option
  5. Compatibility with other plug-ins |session-compatibility-with-other-plug-ins|
  6. Function reference                             |session-function-reference|
   1. Public API for the vim-session plug-in |public-api-for-vim-session-plug-in|
@@ -409,6 +410,15 @@ instead of the default session. Note that the session plug-in will still show
 you the dialog asking whether you want to restore the last used session. To get
 rid of the dialog you have to set |g:session_autoload| to "'yes'".
 
+-------------------------------------------------------------------------------
+The *g:session_persist_font* option
+
+By default the plug-in will save the GUI font with the session to be reused the
+next time that session is loaded, this can be disabled by adding the following
+line to your |vimrc| script:
+>
+  :let g:session_persist_font = 0
+<
 -------------------------------------------------------------------------------
 The *g:session_persist_colors* option