Suport Grayscale, 216 and 256 color output modes.

Ported from the C version of Termbox.
Allow switching output modes in _demos/output.go.

Right and left arrows switch output mode,
up and down arrows similarly.
Better display colourful output modes in _demos/output.go.
SetOutputMode always sets and returns OutputNormal on Windows.
Fix bold and underline for 256 colour output demo.
_demos/output.go: Sync() after changing output mode.
Fix typo in send_attr.

As noted by the initial committer it seems like a bug.