Add way to stop macvim color scheme from loading

Syntax highlight colors that are manually set would get clobbered by the
macvim color scheme which is loaded by default in the global gvimrc.
This loading can now be disabled by adding the line
    let macvim_skip_colorscheme=1
to ~/.vimrc (it will not work in ~/.gvimrc).

src/MacVim/gvimrc

@@ -21,8 +21,11 @@ if !exists("syntax_on")
   syntax on
 endif
 
-if !exists("colors_name")
-    " Use the macvim color scheme by default
+" Load the MacVim color scheme.  This can be disabled by loading another color
+" scheme with the :colorscheme command, or by adding the line
+"   let macvim_skip_colorscheme=1
+" to ~/.vimrc.
+if !exists("macvim_skip_colorscheme") && !exists("colors_name")
     colorscheme macvim
 endif
 
@@ -38,8 +41,7 @@ set printexpr=system('open\ -a\ Preview\ '.v:fname_in)\ +\ v:shell_error
 " This is so that HIG Cmd and Option movement mappings can be disabled by
 " adding the line
 "   let macvim_skip_cmd_opt_movement = 1
-" to the user .vimrc
-"
+" to ~/.vimrc.
 if !exists("macvim_skip_cmd_opt_movement")
   no   <D-Left>       <Home>
   no!  <D-Left>       <Home>
@@ -69,8 +71,7 @@ endif " !exists("macvim_skip_cmd_opt_movement")
 " This is so that the HIG shift movement related settings can be enabled by
 " adding the line
 "   let macvim_hig_shift_movement = 1
-" to the user .vimrc (not .gvimrc!).
-"
+" to ~/.vimrc.
 if exists("macvim_hig_shift_movement")
   " Shift + special movement key (<S-Left>, etc.) and mouse starts insert mode
   set selectmode=mouse,key