Introduce highlight group "GrepperPrompt"

Previously one had to change Question to change the colors for the prompt.

Now GrepperPrompt is used instead. It's linked to Question by default, so users
won't notice the change.

This allows to change the colors for the prompt without touching Question.

doc/grepper.txt

@@ -521,9 +521,9 @@ COLORS                                                          *grepper-colors*
 
 Prompt:~
 
-    Default highlight group: |hl-Question|
+    Default highlight group: GrepperPrompt linked to |hl-Question|.
 >
-    highlight Question ctermfg=160 guifg=#ff0000 cterm=NONE
+    highlight GrepperPrompt ctermfg=160 guifg=#ff0000 cterm=NONE
 >
 Error messages:~
 

plugin/grepper.vim

@@ -3,6 +3,8 @@
 " Escaping test line:
 " ..ad\\f40+$':-# @=,!;%^&&*()_{}/ /4304\'""?`9$343%$ ^adfadf[ad)[(
 
+highlight default link GrepperPrompt Question
+
 "
 " Default values that get used for missing values in g:grepper.
 "
@@ -644,7 +646,7 @@ function! s:prompt(flags)
   "   'cr':         start searching
   let s:prompt_op = 'cancelled'
 
-  echohl Question
+  echohl GrepperPrompt
   call inputsave()
 
   try