Add parameter to let the portals keep running in background.

[axlan]

This is a really simple mod I made to support using this project both for it's Wifi management, and also as a general configuration page. To do this I run the class in non-blocking mode, and I want the config portal to keep running even after the WiFi connection completes successfully. I also made the corresponding issue: #1327 .

[axlan]

I added some additional commits I needed to support my particular use case ( https://github.com/axlan/extended-wifimanager ) . You may not want the parts that change the portal shutdown behavior, but I found using a std::unique_ptr for the WiFiManagerParameter memory management, made the interface a lot less error prone (I was hitting bugs when my vector of parameters would do move constructors).