Avoid blocking user input while reading device output

Having a background thread that just queues everything, read from the mbed,
and echos printable characters allows to keep user input queue alive, which
allows interrupting long-running processes by sending the stop command.
Allow more time when reading and writing flash [in line with javalogo]
Combine stop command and clear com into stop_everything
Start communicating with mbed immediately upon startup

This is useful to pick an output from running mbed.
Add support to dump memory regions, similar to javalogo's dump

This is the first time when Reader.echo became really useful.