Switch istanbul CLI to more recent nyc one

`nyc` is made by the Istanbul people and is meant to integrate nicely with Babel.
Note that at the moment this is just a drop-in replacement of what we currently have in order to fix broken `npm run coverage`, therefore it does not support our recent ES6 stuff.

.gitignore

@@ -1,6 +1,7 @@
 node_modules/
 npm-debug.log
 
+.nyc_output/
 coverage/
 
 # Built assets created at npm install/prepublish time

.nycrc

@@ -0,0 +1,14 @@
+{
+  "all": true,
+  "exclude": [
+    "client/js/bundle.js",
+    "client/js/bundle.vendor.js",
+    "coverage/",
+    "test/"
+  ],
+  "reporter": [
+    "lcov",
+    "text",
+    "text-summary"
+  ]
+}

package.json

@@ -12,7 +12,7 @@
   },
   "homepage": "https://thelounge.github.io/",
   "scripts": {
-    "coverage": "istanbul cover node_modules/mocha/bin/_mocha",
+    "coverage": "nyc mocha",
     "start": "node index",
     "start-dev": "npm-run-all --parallel watch start",
     "build": "npm-run-all build:*",
@@ -66,12 +66,12 @@
     "font-awesome": "4.7.0",
     "handlebars": "4.0.6",
     "handlebars-loader": "1.4.0",
-    "istanbul": "0.4.5",
     "jquery": "2.1.1",
     "jquery-ui": "1.12.1",
     "mocha": "3.2.0",
     "mousetrap": "1.4.6",
     "npm-run-all": "3.1.2",
+    "nyc": "10.0.0",
     "socket.io-client": "1.7.2",
     "stylelint": "7.7.0",
     "urijs": "1.16.1",