Add advanced !packet command

[diamondo25]

This command supports the following 'placeholders':

• lNNN for writing int64
• iNNN for writing uint32
• sNNN for writing uint16
• bNNN for writing uint8
• "text" for writing a string

Also i fixed/expanded the atoli() function. It now acts more like atoi, where it has to remove whitespaces (using isspace) first, then check for the plus/minus sign, and after that finally parse the numbers. After that, make the value negative, of course.