Add deprecation warnings for deprecated options

Closes #114

Author: losingkeys

autoload/ag.vim

@@ -1,17 +1,26 @@
 " NOTE: You must, of course, install ag / the_silver_searcher
 
-" FIXME: Delete deprecated options below on or after 15-7 (6 months from when they were changed) {{{
+" FIXME: Delete deprecated options below on or after 2016-4 (6 months from when the deprecation warning was added) {{{
 
 if exists("g:agprg")
   let g:ag_prg = g:agprg
+  echohl WarningMsg
+  call input('g:agprg is deprecated and will be removed. Please use g:ag_prg')
+  echohl None
 endif
 
 if exists("g:aghighlight")
   let g:ag_highlight = g:aghighlight
+  echohl WarningMsg
+  call input('g:aghighlight is deprecated and will be removed. Please use g:ag_highlight')
+  echohl None
 endif
 
 if exists("g:agformat")
   let g:ag_format = g:agformat
+  echohl WarningMsg
+  call input('g:agformat is deprecated and will be removed. Please use g:ag_format')
+  echohl None
 endif
 
 " }}} FIXME: Delete the deprecated options above on or after 15-7 (6 months from when they were changed)