Implement property getter/setter for context.newline, and make tubes honor it

[heapcrash]

The default newline can now be set context-wide, or per-tube.

>>> t = tube()
>>> t.newline = 'X'
>>> t.unrecv('A\nB\nCX')
>>> t.recvline()
b'A\nB\nCX'

>>> t = tube()
>>> context.newline = '\r\n'
>>> t.newline
b'\r\n'