Add a nice little progress bar while downloading binary

Forked from #1649.

This PR adds a small ascii progress bar to the binary download.
Stalled installations make up a steady background noise of our
issues. It's my hope that by making the binary fetch more
transparent we can stem the tide of those issues.

In order to address @saper's [concerns][1] this progress bar will
respect npm's `progress` config flag.

Downloading
```
Start downloading binary at https://github.com/sass/node-sass/releases/download/v3.8.0/darwin-x64-48_binding.node
Total 2602136 [===============          ] 1566256 60% 4.9s
```
Success
```
Start downloading binary at https://github.com/sass/node-sass/releases/download/v3.8.0/darwin-x64-48_binding.node
Total 2602136 [=========================] 2602136 100% 0.0s
Binary downloaded and installed at /tmp/node-sass/vendor/darwin-x64-48/binding.node
```

[1]: #1649 (comment)

package.json

@@ -64,6 +64,7 @@
     "mkdirp": "^0.5.1",
     "nan": "^2.3.2",
     "node-gyp": "^3.3.1",
+    "progress": "^1.1.8",
     "request": "^2.61.0",
     "sass-graph": "^2.1.1"
   },

scripts/install.js

@@ -8,6 +8,7 @@ var fs = require('fs'),
   path = require('path'),
   sass = require('../lib/extensions'),
   request = require('request'),
+  ProgressBar = require('progress'),
   pkg = require('../package.json');
 
 /**
@@ -56,6 +57,8 @@ function download(url, dest, cb) {
     }
   };
 
+  console.log('Start downloading binary at', url);
+
   try {
     request(url, options, function(err, response) {
       if (err) {
@@ -70,6 +73,19 @@ function download(url, dest, cb) {
       if (successful(response)) {
         response.pipe(fs.createWriteStream(dest));
       }
+
+      if (process.env.npm_config_progress) {
+        var bar = new ProgressBar('Total :total [:bar] :current :percent :etas', {
+          complete: '=',
+          incomplete: ' ',
+          width: 25,
+          total: parseInt(response.headers['content-length'])
+        });
+
+        response.on('data', function(chunk) {
+          bar.tick(chunk.length);
+        });
+      }
     });
   } catch (err) {
     cb(err);