We use a simplified WSPassthroughStream stream to handle any
non-trivial escape sequence (whitespace like newlines and tabs are
passed through).

Then, the stream will call to the event handlers of a simplified
LinearScreen.

This screen will not emulate a 2D screen but just a linear list
of data chunks received from the stream and it will emulate only the
most basic escape sequences and ignore the rest.

The combination of WSPassthroughStream and LinearScreen in processing
the data read from the interpreter *before* being processed by pexpect
makes possible the filtering without breaking the pexpect regex/prompt
matching, nor breaking pre-existent examples that depend on whitespace
nor adds any significant overhead to the runtime.

This filtering can be optionally disabled with `-filter-esc-seqs`. For
terminals other than `dumb`, no filtering is applied.

See #180

… unhandled esc sequences