Allow user to assign a static IP address on ESP boards

[John Maloney]

geek has requested a way to assign a static IP address to an ESP board. The user would also need to provide other information, such as the address of the router and the subnet mask, although we might be able to deduce the subnet mask. See:

https://circuits4you.com/2018/03/09/esp8266-static-ip-address-arduino-example/

[Bernat Romagosa]

Allow static IP on ESP boards (closes #138)

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