Adds npm ignore file.

.npmignore

@@ -0,0 +1,23 @@
+#  .npmignore - Npm ignore configuration.
+#  Copyright (c) 2018 - 2019  Richard Huang <[email protected]>
+#
+#  This program is free software: you can redistribute it and/or modify
+#  it under the terms of the GNU Affero General Public License as
+#  published by the Free Software Foundation, either version 3 of the
+#  License, or (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU Affero General Public License for more details.
+#
+#  You should have received a copy of the GNU Affero General Public License
+#  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+.editorconfig
+.eslintrc.js
+.readme.hbs
+.travis.yml
+CHANGELOG.md
+coverage
+__tests__