onBinary event

[jerch]

Implements an onBinary event to be used to send non UTF-8 compatible data to the backend. Currently only used for DEFAULT mouse reports.

The event data is a string, but its important to treat it as a binary string, example with node-pty:

term.onBinary(data => pty.write(Buffer.from(data, 'binary')));

in difference to the normal UTF8 data handling:

term.onData(data => pty.write(data));
// equivalent to
term.onData(data => pty.write(Buffer.from(data, 'utf-8')));

Closes #2545, fixes #1962.

Reminder: Add note to encoding article when done.