Allow game detection through Steam

Right now, Discord does basic game detection by looking for running processes.  Unfortunately, that doesn't always work.  For example, it doesn't work as well on Linux.

Discord already supports connecting to Steam, to show your Steam Name in your profile.  My request is: Using the Steam user information from that connection, leverage Steam's API to look up the game they are playing.

More specifically, you can use the Steam user's steamID and the GetPlayerSummaries call (https://developer.valvesoftware.com/wiki/Steam_Web_API#GetPlayerSummaries_.28v0002.29).  The game the user is playing is in the `gameextrainfo` field.

This should be an option that can be toggled on or off in the Discord client, so that the user won't have multiple Discord clients trying to simultaneously query Steam.