steam-api: implemented missing SteamStates

There are two "uncommon" SteamStates that are missing, one for friends
"looking to trade," and one for friends "looking to play." These states
have been implemented as TRADE and PLAY.
steam-api: do not explicitly specify the data format

By default, the Steam Web API uses JSON as the data format, which is
what this plugin requires. There is no need for explicitly defining
default behavior.
steam: use account tag instead of account index

Currently, the account index is being used for referencing the Steam
account within command syntax. While this has never been an issue,
using the account tag is cleaner and more reliable.
README: removed outdated resources