Download files from slack and reupload to mattermost (slack/mattermost). Closes #255

Refactor message.Extra to a map[string][]interface{} to have a bit more flexibility
for stuffing extra stuff.

For attached files from slack, files < 1MB size get downloaded (in memory), and get
put into Extra["file"][]config.FileInfo (containing a pointer to the buffer and
the filename). This is not async so slack channels with lots of attached files
may suffer a slowdown. (the download timeout is set at 5 seconds).

bridge/config/config.go

@@ -29,7 +29,12 @@ type Message struct {
 	Gateway   string    `json:"gateway"`
 	Timestamp time.Time `json:"timestamp"`
 	ID        string    `json:"id"`
-	Extra     []interface{}
+	Extra     map[string][]interface{}
+}
+
+type FileInfo struct {
+	Name string
+	Data *[]byte
 }
 
 type ChannelInfo struct {