New Mode: pgnPlay

[mliebelt]

Add code so that it is possible to have 2 players using the pgn-viewer (only) to play against each other on the internet. This should be possible in using WebRTC (now supported by all major browsers), without a backend needed. On that base, it should be more easy possible to implement something like a playing server more easily.

The following features should be supported:

• Starting the pgn-viewer in play mode, for one color, or for both colors.
• Returning enough information on demand how to connect to the other player.
• Automatically send each move to the other player, which will be shown then there.
• (optional) Recording the time spent on moves, and sending that information to the other player.