Set search tools for Grepper (#3069)

- set default search tools available to Grepper (before filtering)
  to be the same as the tools available to FlyGrep

config/plugins/vim-grepper.vim

@@ -0,0 +1,9 @@
+" initialize g:grepper if it does not exist
+if !exists('g:grepper')
+  let g:grepper = {}
+endif
+
+" set value of g:grepper['tools'] to the same list of tools used by FlyGrep
+if !has_key(g:grepper, 'tools')
+  let g:grepper = {'tools': g:spacevim_search_tools}
+endif