Add plugin for VGTV.

This plugin adds support for Norwegian newspaper VG Nett's VGTV. It is heavily inspired by the plugin for Aftonbladet, since they seem to be at least partially based on the same solution. There's the usual support for HDS (as long as streams aren't encrypted) and HLS, as well as plain HTTP streams when those are available.

Tried and tested with a range of different content (also live streams), and seems to work quite well.