Clean and update build output more intelligently

Fixes #183.
Also this and [this line](https://github.com/palantir/eclipse-typescript/pull/184/files#diff-f9d67db69e01e08c8ab96df7a99cbf5eR263) aren't really all that related to the PR (except that they're consistent with [this new line](https://github.com/palantir/eclipse-typescript/pull/184/files#diff-cc510c3b4b1d816bb89f013d5991a1deR148)). Should I split them into a separate commit and/or PR?