Allow "G", "M" and 'k' when parsing arguments

[UweBonnes]

openFPGALoader uses cxxopts for argument parsing. To dump the content from some flash chip, a size argument must be given. At resent I must use "--file-size 1000000" to read 1MByte, What would it take to understand "--file-size 1M"? At the moment leaving "M" and "Mi" asinde ;-)

[jarro2783]

I think this should be up to the application to deal with. Although you can implement this yourself using a custom type.

You can overload cxxopts::parse_value to add a parser for a custom type.

[nigels-com]

That would be nice to have out of the box. Both SI powers of 10 and computer powers of 2.