Frame does not allow partial reads

[tjgq]

Presently, ReadFrame can only read a whole frame in one go. Users that want to track scan progress (e.g., in order to update a progress bar) are forced to use the low-level interface, which is tedious.

Proposed solution:

• A new method Conn.NewFrame that initiates a scan (Conn.Start) and returns a new Frame object before reading any data.
• A new method Frame.ReadMore that performs a single read and updates the object state.
• A new method Frame.ReadAll, for convenience.
• Attributes LinesRead (int) and Complete (bool) on Frame; we need to signal completion separately from number of lines scanned, because Height may not be known beforehand.