chore: switch to files attribute from npmignore (#613)

Prefer the `files` attribute over `.npmignore` to minimize the bytes users
download when installing ShellJS. This cuts the size down from ~1.8M to ~1.2M.

.npmignore

@@ -1,13 +0,0 @@
-test/
-tmp/
-.github/
-.documentup.json
-.gitignore
-.eslintrc
-.eslintignore
-.lgtm
-.travis.yml
-.gitattributes
-appveyor.yml
-RELEASE.md
-CONTRIBUTING.md

package.json

@@ -23,6 +23,14 @@
   "license": "BSD-3-Clause",
   "homepage": "http://github.com/shelljs/shelljs",
   "main": "./shell.js",
+  "files": [
+    "global.js",
+    "make.js",
+    "plugin.js",
+    "shell.js",
+    "bin",
+    "src"
+  ],
   "scripts": {
     "posttest": "npm run lint",
     "test": "node scripts/run-tests && ava --serial ava-test/*.js",