On start support (#324)

* annotating APIs to support onstart

* missed file

* updated templates

* adding new lines in main.ts

* fixing docs

* removing onstart

* updated docs

* updated various docs

* more docs

* adding upgrade policies for blocks

* updated pxt reference

* placing on start under "basic"

docs/blocks/loops/for.md

@@ -4,14 +4,21 @@
  
 Run part of the program the number of times you say.
 
+```block
+for(let i = 0; i <= 4; ++i) {
+}
+```
+
 ### Example: Count to 4
 
 This program will show the numbers 0, 1, 2, 3, and 4 one after another on the LED screen.
 
 ```blocks
-for(let i = 0; i < 5; ++i) {
-basic.showNumber(i)
-}
+input.onButtonPressed(Button.A, () => {
+    for(let i = 0; i < 5; ++i) {
+        basic.showNumber(i)
+    }
+})
 ```
 
 ### See also