WIP: Add option for a custom log proc.

If `:log_proc` is passed, then rest-client will not output any log
messages itself, but will instead call the proc with an event type and a
hash of data that should be useful in outputting log messages.

Folks can use this to implement log levels, redaction, etc.

lib/restclient/request.rb

@@ -138,6 +138,8 @@ module RestClient
       end
 
       @log = args[:log]
+      @log_proc = args[:log_proc]
+
       @tf = nil # If you are a raw request, this is your tempfile
       @max_redirects = args[:max_redirects] || 10
       @processed_headers = make_headers headers
@@ -540,6 +542,11 @@ module RestClient
     def log_request
       return unless log
 
+      if @log_proc
+        @log_proc.call(:request, {request: self, headers: processed_headers})
+        return
+      end
+
       out = []
 
       out << "RestClient.#{method} #{redacted_url.inspect}"
@@ -551,6 +558,11 @@ module RestClient
     def log_response res
       return unless log
 
+      if @log_proc
+        @log_proc.call(:request, {request: self, response: res})
+        return
+      end
+
       size = if @raw_response
                File.size(@tf.path)
              else
@@ -790,6 +802,7 @@ module RestClient
           size += chunk.size
           if log
             if size == 0
+              # TODO XXX
               log << "%s %s done (0 length file)\n" % [@method, @url]
             elsif total == 0
               log << "%s %s (zero content length)\n" % [@method, @url]