PR #59 with enhancements and fixes (Changes done by @JoniVR):

- Code migration to **Swift 4.2**
- **Fixed** the **MonitorControl scheme** which was gone for me
- **Updated the screenshots and readme**
- **Added a feature that detects the default sound output device and only intercepts when an external display is set as default output**, so Airpods, internal macbook volume and other non-monitor devices will be ignored (#48, #15)
- **Fixed app unresponsiveness and high CPU on launch** (might be related to #50, #37, please do keep in mind that on initial launch there will still be a little unresponsiveness because the underlying framework - called [ddcctl](https://github.com/kfix/ddcctl) - attempts to read your volume settings 10 times)
- Added a **version and build number** to the main preferences panel:
![menugeneral](https://user-images.githubusercontent.com/7591717/51052474-0c4c7500-15d7-11e9-883d-804051577bc7.png)
- Added a build script that increases the build number on each run. Version number still needs to be set manually.
- refactored some parts of the code to make it a bit more readable.