Add a "latest" link to downloads.arduino.cc

It would be convenient to be able to download the latest version of the Arduino IDE/tools via a fixed URL for use from a Travis Continuous Integration build. For example, [the Software page](https://www.arduino.cc/en/Main/Software) gets updated to link to the latest release by number (e.g. 1.8.5 today), but that isn't easy for a script to figure out.

Please add a link that is updated each time there is a new release. For example, instead of these two links:
```
https://www.arduino.cc/download_handler.php?f=/arduino-1.8.5-linux64.tar.xz
https://downloads.arduino.cc/arduino-1.8.5-linux64.tar.xz
```
It would be most convenient if these links existed:
```
https://www.arduino.cc/download_handler.php?f=/arduino-latest-release-linux64.tar.xz
https://downloads.arduino.cc/arduino-latest-release-linux64.tar.xz
```