Feature Request: Add SLIP support to LWIP

### Basic Infos

- [ ] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [X] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [ ] I have tested that the issue is present in current master branch (aka latest git).
- [X] I have searched the issue tracker for a similar issue.
- [ ] If there is a stack dump, I have decoded it.
- [ ] I have filled out all fields below.

#### Platform

- Hardware: ESP-12

### Problem Description

I wish to use the SLIP protocol using an ESP8266 and the Arduino IDE, however LWIP for this repository does not have the SLIP code that does come with the standard ESP library found here: https://github.com/espressif/esp-lwip/blob/2.1.2-esp/src/netif/slipif.c

I am asking if you could please add this support so that I can continue development of my project in the Arduino ecosystem.