Add capability to warn with certain programs

Findstr is a buggy program and it doesn't pass even the most simple tests when
used outside of EasyGrep. In particular, it's regular expression evaluation
appears to be broken and its recursive search is bad too. I will likely remove
support for this in a future commit.

Author: dkprice

plugin/EasyGrep.vim

@@ -2077,6 +2077,10 @@ endfunction
 " CheckCommandRequirements {{{
 function! s:CheckCommandRequirements()
     call s:SanitizeMode()
+    if s:CommandHasLen("opt_str_warnonuse")
+        let commandParams = s:GetGrepCommandParameters()
+        call s:Warning(commandParams["opt_str_warnonuse"])
+    endif
 endfunction
 " }}}
 " Extension Tracking {{{
@@ -2687,6 +2691,7 @@ function! s:ConfigureGrepCommandParameters()
                 \ 'req_str_recurse': '/S',
                 \ 'req_str_caseignore': '/I',
                 \ 'req_str_casematch': '',
+                \ 'opt_str_warnonuse': 'The findstr program is buggy and not-recommended for use',
                 \ 'opt_str_patternprefix': '',
                 \ 'opt_str_patternpostfix': '',
                 \ 'opt_str_wholewordprefix': '"\<',