Add request placeholder support for querying request cookies. (#1392)

* Add request placeholder support for querying request cookies.

This adds the ability to query the request cookies for placeholders
using the syntax "@cookiename".

For example, this would allow rewriting based on a cookie:
  rewrite {
    if @version is 'dev'
    to /dev/index.html
  }

* Switch cookie special char from @ to :

* Switch special char for cookies from : to ~

caddyhttp/httpserver/replacer.go

@@ -198,6 +198,13 @@ func (r *replacer) getSubstitution(key string) string {
 			}
 		}
 	}
+	// next check for cookies
+	if key[1] == '~' {
+		name := key[2 : len(key)-1]
+		if cookie, err := r.request.Cookie(name); err == nil {
+			return cookie.Value
+		}
+	}
 
 	// search default replacements in the end
 	switch key {