Add another HexFormattingOption that adds a '0x' prefix to the returned string

[credfeto]

e.g:

        /// <summary>
        /// Produces a string with a '0x' prefix.
        /// </summary>
        AddHexPrefix = 4

So instead of producing say:

123456789abcdef0

would produce

0x123456789abcdef0

Lots of code we've got requires a 0x prefix and that means we have to do an extra allocation using HexMate to our own routines.

Happy to have a go at producing a PR for this.