Allow changing the Telnet buffer size

Currently it's hard-coded to be [1024](https://github.com/hneemann/Digital/blob/932791eb6486d04f2ea938d83bcdb71b56d3a3f6/src/main/java/de/neemann/digital/core/io/telnet/Server.java#L29), it could be nice if this was settable in the settings of the Telnet component.