Allow features to be disabled at build time (#143)

* Add framework for built-time feature selection
* Allow MQTT, NTP, OTA features to be disabled at build time
* Allow Project screens to be disabled at build time
* Allow security features to be disabled at build time
* Switch to std::function for StatefulService function aliases for greater flexibility
* Bump various UI lib versions
* Update docs

features.ini

@@ -0,0 +1,7 @@
+[features]
+build_flags = 
+    -D FT_PROJECT=1
+    -D FT_SECURITY=1
+    -D FT_MQTT=1
+    -D FT_NTP=1
+    -D FT_OTA=1