Allow command line settings to be stored in file

[djhworld]

At the moment you have to start the emulator with a bunch of command line flags

I often use the same ones every time (e.g. -noboot and -size 2)

Would be nice to be able to store these into a file that is parsed at startup, with any flags provided on the command line acting as overrides.

Could also use this file to store stuff like the keyboard layout settings as well

This could possibly be combined into #19. with the settings stored in a database rather than a file. Although that would mean I would have to write something to allow you to maintain the database too....

[djhworld]

Done, settings can now be stored in a JSON file (see commit e1e6e88)

If flags are provided on the command line then they take precedence over whatever is in the settings file