Use 0x10 rather than 0xff for the default colour.

In 256 colour modes, colour number 16 is redundant (same as 0).
256 colour support.
Use enums instead of literals.
Don't allow stray ? inside ESC [.
Detect and ignore OSC codes (window title, palette change, etc).

Control characters inside the passed string are not yet supported: they go to
the terminal like it's the case with every other ECMA-48 code.  FIXME.
Release 0.17