SoftwareSerial on ESP boards

[avaldebe]

Fix broken SoftwareSerial on ESP8266, close #6
Add SoftwareSerial support for the ESP32, see #7

[avaldebe]

No SoftwareSerial for the ESP32

ESPSoftwareSerial on a MH ET LIVE ESP32MiniKit failed catastrophically

[env:esp32minikit]
platform = espressif32
lib_deps = EspSoftwareSerial@>=6.7.1
board = mhetesp32minikit
build_flags = -D PMS_RX=21 -D PMS_TX=17

PMS sensor on SWSerial
  RX:21
  TX:17
Guru Meditation Error: Core  1 panic'ed (LoadProhibited). Exception was unhandled.
Core 1 register dump:
PC      : 0x400d18f0  PS      : 0x00060130  A0      : 0x800d0d6d  A1      : 0x3ffb1f50  
A2      : 0x3ffc00b4  A3      : 0x00000002  A4      : 0x0000000a  A5      : 0x00000003  
A6      : 0x00000001  A7      : 0x00000000  A8      : 0xfefefefe  A9      : 0x3ffb1f20  
A10     : 0x3ffe3b4c  A11     : 0x3f401053  A12     : 0x00000007  A13     : 0x00000001  
A14     : 0x00000001  A15     : 0x00000040  SAR     : 0x00000001  EXCCAUSE: 0x0000001c  
EXCVADDR: 0xfefeff0a  LBEG    : 0x400014fd  LEND    : 0x4000150d  LCOUNT  : 0xffffffff  

Backtrace: 0x400d18f0:0x3ffb1f50 0x400d0d6a:0x3ffb1f80 0x400d30b3:0x3
--- exit ---