Several additions both to colors and to commands

[buzden]

I propose several additions, for ease of review I made each change in a separate commit:

• add commands for saving/restoring cursor position;
• add commands for turning on/off cursor symbol;
• add probably forgotten turning off for existing italic mode;
• add support for 8-bit and 24-bit ANSI colours;
• add couple of commonly used aliases for styles.

Proposed cursor-related commands are not part of the official standard, but are supported even wider than present in the library features like double underline and rapid blinking. So, I think, it's okay for use in this library.