Add support for a 'configuration setup'

[MBilderbeek]

We know have a way to start with any MSX, but it's always empty. On a real MSX setup, you often have several extensions already plugged in, even when it's off. And when you turn it on, the extensions are directly available.

Something similar would be good to have in openMSX as well: have a way to start openMSX with a specific configuration, i.e. a certain MSX with certain extensions inserted. (Technically, one could also include disks inserted, and tapes... as these would also stay in when you power off your real MSX and later turn it back on... but this may be a bit uncomfortable.)

Such a configuration is just a combination of -machine and -extX command line options. So, just one level higher.

This would make it much easier to start a specific MSX set up, as you have it at home.... without having to use a workaround (e.g. a savestate or creating your own XML; all having their disadvantages.)