pxt-microbit can now be required and exports useful information about its bundled pxt-core (#305)

cmds/pxtrequire.ts

@@ -0,0 +1,4 @@
+import * as path from "path";
+export let pxtCore = require("pxt-core");
+// require.resolve() gives path to [pxt dir]/built/pxt.js, so move up twice to get pxt root dir
+export let pxtCoreDir = path.resolve(require.resolve("pxt-core"), "..", "..");