Don't include JetBrains settings in npm distribution

Remove `.idea` from the npm package. Also ignore `.idea/workspace.xml`
and `.idea/tasks.xml` in git, per [JetBrains support][].

[JetBrains support]: https://intellij-support.jetbrains.com/entries/23393067)
Merge pull request #4 from rouge8/master

Don't include JetBrains settings in npm distribution