Enable install for node-chakracore

Today, `npm install` downloads the pre-built `binding.node` binary from [node-sass repo](https://github.com/sass/node-sass/releases).
However these binaries are compiled using `nodev8` and an attempt to use it with [node-chakracore](https://github.com/nodejs/node-chakracore) would fail.
One option is to have `node-sass` host binaries pre-built using node-chakracore and during installation have a way to detect the engine (using `process.jsEngine')
and download the appropriate engine compliant binary. I have opened #1776 to start the discussion for this.

For now, as an interim solution, I have disabled downloading the pre-built binary if running with node-chakracore. In `postinstall` stage, it would see that the binary
is not present and then would build the binary (only one-time) with node-chakracore.

scripts/install.js

@@ -106,6 +106,11 @@ function checkAndDownloadBinary() {
   if (sass.hasBinary(sass.getBinaryPath())) {
     return;
   }
+
+  if (process.jsEngine && process.jsEngine === 'chakracore') {
+      console.log('Installing using node-chakracore. Need rebuild of binary.');
+      return;
+  }
 
   mkdir(path.dirname(sass.getBinaryPath()), function(err) {
     if (err) {