Add keywords to package.json

I noticed we have no search tags on npmjs.com

Author: Tyriar

package.json

@@ -7,6 +7,21 @@
   "types": "typings/xterm.d.ts",
   "repository": "https://github.com/xtermjs/xterm.js",
   "license": "MIT",
+  "keywords": [
+    "cli",
+    "command-line",
+    "console",
+    "pty",
+    "shell",
+    "ssh",
+    "styles",
+    "terminal-emulator",
+    "terminal",
+    "tty",
+    "vt100",
+    "webgl",
+    "xterm"
+  ],
   "scripts": {
     "prepackage": "npm run build",
     "package": "webpack",