Adapted for Kirby 3

The plugin was adapted to the new plugin architecture of Kirby 3.

Breaking Changes

The configuration options now have an official way of namespacing them, so they were changed:

• tags.download.class was changed to jbeyerstedt.download.class.
• tags.download.warnings was changed to jbeyerstedt.download.warnings

Composer Support

Composer support should work, but is untested. Please feel free to improve this (see #5).