Add option to turn off vim-latex

doc/latex.txt

@@ -187,6 +187,7 @@ descriptions also list the default values.
 
 Overview:~
 
+  |g:latex_enabled|
   |g:latex_build_dir|
   |g:latex_complete_close_braces|
   |g:latex_complete_enabled|
@@ -220,6 +221,10 @@ Overview:~
 ------------------------------------------------------------------------------
 Detailed descriptions and default values:~
 
+                                                             *g:latex_enabled*
+If this variable exists and is 0, then |vim-latex| is completely disabled.  By
+default, it is not defined.
+
                                                            *g:latex_build_dir*
 Set this variable in case a dedicated build dir is used with latexmk/latex
 compilations. >

ftplugin/tex.vim

@@ -4,6 +4,10 @@
 " Email:      karl.yngve@gmail.com
 "
 
+if exists('g:latex_enabled') && !g:latex_enabled
+  finish
+endif
+
 if exists('b:did_ftplugin')
   finish
 endif