xterm compat audit: cursor left (CUB)

[mitchellh]

This implements full compatibility with xterm for CUB (backspace and CSI D). I found numerous bugs or missing features in our implementation that are now fixed:

• Implement mode 45 (reverse wrap): cursor left wraps back up a row for soft-wrapped text
• Implement mode 1045 (extended reverse wrap): cursor left wraps back up for all rows and will wrap around to the bottom of the scroll region once at the top
• Scroll region awareness, including left/right even though we don't allow changing that yet
• Test cases for all functionality