Fix behavior of \b after line wrap.

[szym]

This fixes issue #49.

[takluyver]

I think this does the wrong thing after an explicit newline, e.g. abc\r\n\bdef. At least in my regular terminal emulator, \b can go backwards if it has wrapped onto a new line, but not after an explicit newline.

I think fixing this would require term.js to keep track of whether a line has wrapped or been explicitly ended. That would also be beneficial for copying and pasting out of a terminal.