Pushbullet support!

Breaking changes

• settings.json and triggers.json can no longer have unrecognized properties in them. While this is technically a breaking change it shouldn't impact anyone in practice. The addition of this requirement is to ensure new users get real-time notifications of typos/mistakes in their configuration files while editing in tools that support schema validation (such as Visual Studio Code). Resolves issue 291.

Non-breaking changes

• Pushbullet notifications are now supported as trigger handlers. Enable it in settings.json then add pushbullet handlers to your triggers in triggers.json. See the wiki for more details. Resolves issue 119.