New Mode: Echo

[hellow554]

Hey kumavale,

did I already mention that I really love this project?

I'm currently connected to an embedded device which has an CLI, but does not echo back the characters I typed in, which means that I don't see what I type in.

Most of the Serial Terminal Programs provide some kind of echo output, where the typed characters are printed on the terminal, regardless whether they have been sent back or not.

The echoed characters should have a different color, typically (light) green.
Also enabling them via arguments or escape sequence (~e) should be possible.

I would like to see such feature. What do you think?

[kumavale]

I think this a very good idea!
getch-rs, which sisterm depends on, provides a function for local echo, so it seems to be possible if we use it.
I think we should add the -e,--echo flags and the ~e escape sequence.