Use platformio's extend feature (#149)

* Use platformio's extend feature

* Pin ESP32 platform version

* Do not use env: prefix for base envs

By using a custom prefix base:, we don't need to list the environments we *want* to build explicitly.

* Fix espressif32 platform version

platformio.ini

@@ -11,7 +11,10 @@
 [platformio]
 src_dir = ESP8266WirelessPrintAsync
 
+
+# common variables shared by the environments
 [common]
+framework = arduino
 lib_deps =
     https://github.com/greiman/SdFat#3b79f38
     https://github.com/me-no-dev/ESPAsyncWebServer#95dedf7
@@ -22,24 +25,37 @@ lib_deps =
     https://github.com/bblanchon/ArduinoJson#3df4efd
     https://github.com/Makuna/NeoPixelBus#9619fef
 
-[env:nodemcuv2]
-platform = espressif8266@2.0.0 ; Corresponds to https://github.com/platformio/platform-espressif8266/releases/tag/v2.0.0 = https://github.com/esp8266/Arduino/releases/tag/2.5.0
-board = nodemcuv2
-framework = arduino
+
+# base environments
+# can be extended by board-specific environments
+[base:esp8266]
+; corresponds to https://github.com/platformio/platform-espressif8266/releases/tag/v2.0.0
+; see https://github.com/esp8266/Arduino/releases/tag/2.5.0
+platform = espressif8266@2.0.0
+framework = ${common.framework}
+lib_deps = ${common.lib_deps}
+
+[base:esp32]
+; corresponds to https://github.com/platformio/platform-espressif32/releases/tag/v1.8.0
+; see https://github.com/espressif/arduino-esp32/releases/tag/1.0.2
+platform = espressif32@1.8.0
+framework = ${common.framework}
 lib_deps =
     ${common.lib_deps}
+    https://github.com/bbx10/Hash_tng
+
+
+[env:nodemcuv2]
+board = nodemcuv2
+extends = base:esp8266
 
 [env:d1_mini]
-platform = espressif8266@2.0.0 ; Corresponds to https://github.com/platformio/platform-espressif8266/releases/tag/v2.0.0 = https://github.com/esp8266/Arduino/releases/tag/2.5.0
 board = d1_mini
-framework = arduino
-lib_deps =
-    ${common.lib_deps}
+extends = base:esp8266
 
+# esp32dev works for the majority of ESP32 based dev boards
+# there are more specific board configurations available, feel free to send PRs adding new ones
 [env:esp32dev]
-platform = espressif32@1.8.0 ; Corresponds to https://github.com/platformio/platform-espressif32/releases/tag/v1.8.0 = https://github.com/espressif/arduino-esp32/releases/tag/1.0.2
 board = esp32dev
-framework = arduino
-lib_deps =
-    ${common.lib_deps}
-    https://github.com/bbx10/Hash_tng
+extends = base:esp32
+