refactor broadcasting to support except and restrict

[hhaensel]

Currently, it is not possible to address single clients by push! or run commands.

To remedy this situation I propose to

• modify existing keyword except to support not only UInt, but also Vetor{UInt} to exclude more than one client
• remove WebSocket as a possible value for except and rather supply the id as it is done in WebThreads
• add a keyword restrict to restrict messages to a client or list of clients

Furthermore, I have removed duplicate code in WebThreads.

There's a corresponding PR in Stipple.