add enable-0x-prefix flag

[gzliudan]

Add a bool flag --enable-0x-prefix. This default value of this flag is false, and can be set to true when startup. The function MarshalText choose address prefix with this flag:

func (a Address) MarshalText() ([]byte, error) {
	// Handle '0x' or 'xdc' prefix here.
	if (Enable0xPrefix) {
		return hexutil.Bytes(a[:]).MarshalText()
	} else {
		return hexutil.Bytes(a[:]).MarshalXDCText()
	}
}

[AnilChinchawale]

LGTM...